Skip to main content

Full text of "Essays upon heredity and kindred biological problems. Authorised translation"

See other formats


j,^--; 



» B\ 



HO*W® 



OST HO'*<f 



\ ISO 



\i 






\ 



Digitized by tine Internet Arciiive 

in 2007 witii funding from 

IVIicrosoft Corporation 



littp://www.arclnive.org/details/essaysuponlierediOOweisuoft 



IV^'I 



TRANSLATIONS 

OF 

FOREIGN BIOLOGICAL MEMOIRS 

IV. 



bonbon 

HENRY FROWDE 




Oxford University Press Warehouse 
Amen Corner, E.G. 



ESSAYS UPON HEREDITY 



AND KINDRED 

BIOLOGICAL PROBLEMS 

BY 

Dr. august WEISMANN 

VROFESSOK IN THK UNIVERSITY OF FREIBURG IN BREISGAU 

AUTHORISED TRANSLATION 

EDITKI) BY 



EDWARD B. POULTON, M.A., F.L.S., F.G.S. 

. TUTOR OF KEBLE COLLEGE, OXFORD 
LKOTURER IN NATURAL SCIENCE, JESUS COLLEGE, OXFORD 



SELMAR SCHONLAND, Ph.D. 

SUK-CURATOR OK THE FIKLniNG HERBARIUM IN THK UNIVEKSITV OF OXFORD 
AND 

ARTHUR E. SHIPLEY, M.A., F.L.S. 

FELLOW AND LECTURER OF CHRIST'S COLLEGE, CAMBRIDGE 
DEMONSTRATOR OF COMPARATIVE ANATOMY IN THE UNIVERSITY OP CAMBRIDGE 



Oxford 

AT THE CLARENDON PRESS 
1889 






(> 



V.I 



^5 



AUTHOE'S PEEFACE. 



The essays which now appear for the first time in the form 
of a single volume were not written upon any prearranged plan, 
but have been published separately at various intervals during 
the course of the last seven years. Although when writing the 
earlier essays I was not aware that the others would follow, the 
whole series is, nevertheless, closely connected together. The 
questions which each essay seeks to explain have all arisen 
gradually out of the subjects treated in the first. Reflecting upon 
the causes which regulate the duration of life in various forms, 
I was drawn on to the consideration of fresh questions which 
demanded further research. These considerations and the results 
of such research form the subject-matter of all the subsequent 
essays. 

I ^m here making use of the word ' research ' in a sense 
somewhat different from that in which it is generally employed 
in natural science ; for it is commonly supposed to imply the 
making of new observations. Some of these essays, especially 
Nos. IV, V, and VI, essentially depend upon new discoveries. 
But in most of the remaining essays the researches are of a 
more abstract nature, and consist in bringing forward new 
points of view, founded upon a variety of well-known facts. 
I believe, however, that the history of science proves that 
advance is not only due to the discovery of new facts, but 
also to their correct interpretation : a true conception of natural 
processes can only be arrived at in this way. It is chiefly in 

b 



VI AUTHORS PREFACE. 

this sense that the contents of these essays are to be looked 
upon as research. 

The fact that they contain the record of research made it 
impossible to introduce any essential alterations in the trans- 
lation, even in those points about which my opinion has since 
changed to some extent. I should to-day express some of the 
points in Essays I, IV, and V, somewhat differently ; but had 
I made such alterations, the relation between the essays as a 
whole would have been rendered less clear, for each of the 
earlier ones formed the foundation of that which succeeded it. 
Even certain errors of interpretation are on this account left 
uncorrected. Thus, for instance, in Essay IV it is assumed that 
the two polar bodies expelled by sexual eggs are identical ; 
for at that time there was no reason for doubting that they 
were physiologically equivalent. The discovery of the numerical 
law of the polar bodies described in Essay VI, led to what I 
believe to be a truer knowledge of them. In this way the 
causes of parthenogenesis, as developed in Essay V, received 
an important addition in the fact published in Essay VI, that 
only one polar body is expelled by pai'thenogenetic eggs. This 
fact alone explains why sexual eggs cannot as a rule develope 
without fertilization. 

Hence the reader must not take the individual essays as the 
full and complete expression of my present opinion ; but they 
must rather be looked upon as stages in research, as steps 
towards a more perfect knowledge. 

I must therefore express the hope that the essays may be read 
in the same order as that in which they appeared, and in which 
they are arranged in the present volume. The reader will then 
follow the same road which I traversed in the development of 
the views here set forth ; and even though he may be now and 
then led away from the direct route, perhaps such deviations 
may not be without interest. 

I should wish to express my warm thanks to Mr. Poulton 
for the great trouble he has taken in editing the translation, 
which in many places presented exceptional difficulties. The 



AUTHOR S PREFACE. Vll 

greater part of the text I have looked through in proof, and I 
believe that it well expresses the sense of the original ; although 
naturally I cannot presume to judge concerning the niceties of 
the English language. I am especially grateful to the three 
gentlemen who have brought these essays before an English 
public, because I believe that many English naturalists, even when 
thoroughly conversant with the German tongue, might possibly 
misinterpret many points in the original; for the difficulty of 
the questions treated of greatly increases the difficulty of the 
language. 

If the readers of this book only feel half as much pleasure in 
its perusal as I experienced in writing it, I shall be more than 
satisfied. 

AUGUST WEISMANN. 

Fbeiburg I. Breisgau, 
January, 1889. 



b2 



Vt^^ 



EDITOES' PREFACE. 



The attention of English biologists and men of science was 
first called to Professor Weismann's essays by an article entitled 
'Death' in 'The Nineteenth Century' for May, 1885, by Mr. 
A. E. Shipley. Since then the interest in the author's argu- 
ments and conclusions has become very general ; having been 
especially increased by Professor Moseley's two articles in 
'Nature' (Vol. XXXIII, p. 154, and Vol. XXXIV, p. 629), and 
by the discussion upon ' The Transmission of Acquired Cha- 
racters,' introduced by Professor Lankester at the meeting of 
the British Association at Manchester in 1887, — a discussion in 
which Professor Weismann himself took part. The deep interest 
which has everywhere been expressed in a subject which con- 
cerns the very foundations of evolution, has encouraged the 
Editors to hope that a volume containing a collection of all 
Professor Weismann's essays upon heredity and kindred problems 
would supply a real want. At the present time, when scientific 
periodicals contain frequent references to these essays, and when 
the various issues which have been raised by them are dis- 
cussed on every occasion at which biologists come together, it 
is above all things necessary to know exactly what the author 
himself has said. And there are many signs that discussion has 
already suffered for want of this knowledge. 

A translation of Essays I and II was commenced by Mr. A. E. 
Shipley during his residence at Freiburg in the winter of 1884. 
His work was greatly aided by the kind assistance of Dr. van 
Rees of Amsterdam, to whom we desire to express our most 
sincere thanks. The translation was laid aside until the 
summer of 1888, when Mr, Shipley was invited to co-operate 



X EDITORS PREFACE. 

with the other editors in the preparation of the present volume ; 
the Clarendon Press having consented to publish the complete 
series of essays as one of their Foreign Biological Memoirs. 

We think it probable that this work may interest many who 
are not trained biologists, but who approach the subject from its 
philosophical or social aspects. Such readers would do well to 
first study Essays I, II, VII, and VIII, inasmuch as some pre- 
paration for the more technical treatment pursued in the other 
essays will thus be gained. 

The notes signed A. W. and dated, were added by the author 
during the progress of the translation. The notes included in 
square brackets were added by the Editors ; the authorship being 
indicated by initials in all cases. 

In conclusion, it is our pleasant duty to thank those who have 
kindly helped us by reading the proof-sheets and making valu- 
able suggestions. Our warmest thanks are due to Mrs. Arthur 
Lyttelton, Mr. W. Hatchett Jackson, Deputy Linacre Professor 
in the University of Oxford, Mr. J. S. Haldane, and Professor R. 
Meldola. Important suggestions were also made by Professor 
E. Ray Lankester, Mr. Francis Galton, and Dr. A. R. Wallace. 
Professor W. N. Parker also greatly helped us by looking over 
the proof-sheets with Professor Weismann. 

E. B. P. 

S. S. 
A. E. S. 

Oxford, February, 1889. 



yi. 



CONTENTS. 



Translator. Title. Page. 

I. A.E.Shipley . . The Duration of Life, 1881 . ". . • i 

11. „ On Heredity, 1883 67 

III. „ Life and Death, 1883 107 

IV. Selmar Sch5nland The Continuity op the Germ-plasm as the 

Foundation op a Theory of Heredity, 1885 161 

V. „ The Significance of Sexual Reproduction in 

THE Theory of Natural Selection, 1886 . 251 

YI. ^j On the Number of Polar Bodies and their 

Significance in Heredity, 1887 . • -333 

VII. ■/ „ On the Supposed Botanical Proofs op the 

Transmission op Acquired Characters,' 1888. 385 

VIII. J „ The Supposed Transmission op Mutilations, 

1888 419 



^> 



Abstracts of Professor Weismann's Essays on Heredity and Kindred 
Problems, already Published in this Country. 



I. A short abstract in ' Nature,' Vol. XXXVII, pp. 541-542, by P. C. Mitchell. 

II. A short abstract in ' Nature,' Vol. XXXVIII, pp. 156-157, by P. C. Mitchell. 

III. A short article on the subject of this Essay in ' The Nineteenth Century ' for 
May, 1885, by A. E. Shipley. 

IV. Abstract in 'Nature,' Vol. XXXIII, pp. 154-157, by Professor Moselby. 

V. Abstract in * Nature,' Vol. XXXIV, pp. 629-632, by Professor M08ELEY. 

VI. Abstract in * Nature,' Vol. XXXVI, pp. 607-609, by Professor Weismann. 

VII, VIII. The Essays being of so recent a date no abstract has yet appeared in this 
country. 

A criticism of Professor Weismann's theories will be found in ' The Physiology of 
Plants,' by Professor Vines, Lecture XXIII, pp. 660 et seqq. 



I. 

THE DUEATION Of LIFE. 

1881. 



THE DURATION OF LIFE, 



PREFACE. 



The following paper was read at the meeting of the Association 
of German Naturalists at Salzburg, on September 2ist, 1881 ; 
and it is here printed in essentially the same form. A somewhat 
longer discussion of a few points has been now intercalated ; these 
were necessarily omitted from the lecture itself for the sake of 
brevity, and are, therefore, not contained in the account printed in 
the Proceedings of the fifty-fourth meeting of the Association. 

Further additions would not have been admissible without an 
essential change of form, and therefore I have not put into the 
text a note which ought otherwise to have been there, and which is 
now to be found in the Appendix, as Note 8. It fi.lls up a gap 
which was left in the 'text, for the above-mentioned reason, by 
attempting to give an explanation of the normal death of cells of 
tissues — an explanation which is required if we are to maintain 
that unicellular organisms are so constituted as to be potentially 
immortal. 

The other parts of the Appendix contain, partly further expan- 
sions, partly proofs of the views brought forward in the text, and 
above all a compilation of all the observations which are known to 
me upon the duration of life in several groups of animals. I am 
indebted to several eminent specialists for the communication of 
many data, which are among the most exact that I have been able 
to obtain. Thus Dr. Hagen of Cambridge (U.S.A.) was kind enough 
to send me an account of his observations upon insects of different 
orders : Mr. W. H. Edwards of West Virginia, and Dr. Speyer of 
Ehoden — their experience with butterflies. Dr. Adler of Schleswig 
sent me data upon the duration of life in Ci/nijndae, which have a 
special value, as they are accompanied by very exact observations 

B 3 



4 THE DURATION OF LIFE. 

upon the conditions of life in these animals ; hence in this case we 
can directly examine the factors upon which, as I believe, the dura- 
tion of life is chiefly based. Sir John Lubbock in England, and 
Dr. August Forel of Zurich, have had the kindness to send me 
an account of their observations upon ants, and S. Clessin of 
Ochsenfurth his researches upon our native land and fresh-water 
Mollusca. 

In publishing these valuable communications, together with all 
facts which I have been able to collect from literature upon the 
subject of the duration of life, and the little which I have myself 
observed upon this subject, I hope to provide a stimulus for 
further observation in this field, which has been hitherto much 
neglected. The views which I have brought forward in this paper 
are based on a comparatively small number of facts, at least as far 
as the duration of life in various species is concerned. The larger 
the number of accurate data which are supplied, and the more 
exactly the dvu-ation of life and its conditions are ascertained, the 
more securely will it be possible to establish our views upon the 
causes which determine the duration of life. 

A. W. 

Naples, Dec. 6, 1881. 



T. 

THE DURATION OF LIFE. 

With your permission, I will bring before you to-day some 
thoughts upon the subject of the duration of life. I can scarcely 
do better than begin with the simple but significant words of 
Johannes Miiller : ' Organic bodies are perishable ; while life main- 
tains the appearance of immortality in the constant succession of 
similai" individuals, the individuals themselves pass away.' 

Omitting, for the time being, any discussion as to the precise 
accuracy of this statement, it is at any rate obvious that the life of 
an individual has its natural limit, at least among those animals 
and plants which are met with in every-day life. But it is equally 
obvious that the limits are very differently placed in the various 
species of animals and plants. These diflPerences are so manifest 
that they have given rise to popular sayings. Thus Jacob Grimm 
mentions an old German saying, ' A wren lives three years, a dog 
three times as long as a wren, a horse three times as long as a dog, 
and a man three times as long as a horse, that is eighty-one years. 
A donkey attains three times the age of a man, a wild goose three 
times that of a donkey, a crow three times that of a wild goose, a 
deer three times that of a crow, and an oak three times the age of 
a deer.' 

If this be true a deer would live 6000 years, and an oak nearly 
20,000 years. The saying is certainly not founded upon exact obser- 
vation, but it becomes true if looked upon as a general statement 
that the duration of life is very different in different organisms. 

The question now arises as to the causes of these great differ- 
ences. How is it that individuals are endowed with the power 
of living long in such very various degrees ? 

One is at first tempted to seek the answer by an appeal to the 
differences in morphological and chemical structure which separate 



6 THE DURATION OF LIFE. 

species from one another. In fact all attempts to throw light upon 
the subject which have been made up to the present time lie in 
this direction. 

All -these explanations are nevertheless insufficient. In a certain 
sense it is true that the causes of the duration of life must be con- 
tained in the organism itself, and cannot be found in any of its 
external conditions or circumstances. But structure and chemical 
composition — in short the physiological constitution of the body in 
the ordinary sense of the words — are not the only factors which 
determine duration of life. This conclusion forces itself upon our 
attention as soon as the attempt is made to explain existing facts 
by these factors alone : there must be some other additional cause 
contained in the organism as an unknown and invisible part of its 
constitution, a cause which determines the duration of life. 

The size of the organism must in the first place be taken into 
consideration. Of all organisms in the world, large trees have the 
longest lives. The Adansonias of the Cape Verd Islands are said 
to live for 6000 years. The largest animals also attain the greatest 
age. Thus there is no doubt that whales live for some hundreds 
of years. Elephants live 300 years, and it would not be difficult 
to construct a descending series of animals in which the duration 
of life diminishes in almost exact proportion to the decrease in the 
size of the body. Thus a horse lives forty years, a blackbird 
eighteen, a mouse six, and many insects only a few days or 
weeks. 

If however the facts are examined a little more closely it will be 
observed that the great age (200 years) reached by an elephant 
is also attained by many smaller animals, such as the pike and 
carp. The horse lives forty years, but so does a cat or a toad ; 
and a sea anemone has been known to live for over fifty years. The 
duration of life in a pig (about twenty years) is the same as that in 
a crayfish, although the latter does not neai-ly attain the hun- 
dredth part of the weight of a pig. 

It is therefore evident that length of life cannot be determined 
by the size of the body alone. There is, however, some relation 
between these two attributes. A large animal lives longer than a 
small one because it is larger ; it would not be able to become even 
comparatively large unless endowed with a compai-atively long dura- 
tion of life. 



THE DURATION OP LIFE. , 7 

Apart from all other reasons, no one could imagine that the 
gig-antic body of an elephant could be built up like that of a mouse 
in three weeks, or dn a single day like that of the larva of certain 
flies. The gestation of an elephant lasts for nearly two years, and 
maturity is only reached after a lapse of about twenty-four years. 

Furthermore, to ensure the preservation of the species, a longer 
time is required by a large animal than by a small one, when both 
have reached maturity. Thus Leuckart and later Herbert Spencer 
have pointed out that the absorbing surface of an animal only in- 
creases as the square of its length, while its size increases as the 
cube ; and it therefore follows that the larger an animal becomes, 
the greater will be the difficulty experienced in assimilating any 
nourishment over and above that which it requires for its own 
needs, and therefore the more slowly will it reproduce itself. 

But although it may be stated generally that the duration of 
the period of growth and length of life are longest in the largest 
animals, it is nevertheless impossible to maintain that there is any 
fixed relation between the two ; and Flourens was mistaken when 
he considered that the length of life was always equivalent to five 
times the duration of the period of growth. Such a conclusion 
might be accepted in thfi case of man if we set his period of growth 
at twenty years and his length of life at a hundred ; but it 
cannot be accepted for the majority of other Mammalia. Thus 
the horse lives from forty to fifty years, and the latter age is at 
least as frequently reached among horses as a hundred years among 
men ; but the horse becomes mature in four years, and the length 
of its life is thus ten or twelve times as long as its period of 
growth. 

The second factor which influences the duration of life is purely 
physiological : it is the rate at which the animal lives, the rapidity 
with which assimilation and the other vital processes take place. 
Upon this point Lotze remarks in his Microcosmus — ' Active and 
restless mobility destroys the organized body : the swift-footed animals 
hunted by man, as also dogs, and even apes, are inferior in length 
of life to man and the larger beasts of prey, which satisfy their needs 
by a few vigorous efforts.' ' The inertness of the Amphibia is, on 
the other hand, accompanied by relatively great length of life.' 

There is certainly some truth in these observations, and yet it 
would be a great mistake to assume that activity necessarily implies 



8 THE DURATION OF LIFE. 

a short life. The most active birds have very long lives, as will 
be shown later on : they live as long as and sometimes longer than 
the majority of Amphibia which reach the same size. The organism 
must not be looked upon as a heap of combustible material, which 
is completely reduced to ashes in a certain time the length of which 
is determined by size, and by the rate at which it burns ; but it 
should be rather compared to a fire, to which fresh fuel can be 
continually added, and which, whether it bums quickly or slowly, 
can be kept burning as long as necessity demands. 

The connection between activity and shortness of life cannot be 
explained by supposing that a more rapid consumption of the 
body occurs, but it is explicable because the increased rate at which 
the vital processes take place permit the more rapid achievement 
of the aim and purpose of life, viz. the attainment of maturity 
and the reproduction of the species. 

When I speak of the aim and purpose of life, I am only using 
figures of speech, and I do not mean to imply that nature is in any 
way working consciously. 

When I was speaking of the relation between duration of life 
and the size of the body, I might have added another factor 
which also exerts some influence, viz. the complexity of the stinic- 
ture. Two organisms of the same size, but belonging to different 
grades of organization, will require different periods of time for 
their development. Certain animals of a very lowly organization, 
such as the Rhizopoda, may attain a diameter of -5 mm. and may 
thus become larger than many insects' eggs. Yet under favourable 
circumstances an Amoeba can divide into two animals in ten 
minutes, while no insect's egg can develope into the yoimg animal 
in a less period than twenty-four hours. Time is required for the 
development of the immense number of cells which must in the 
latter case arise from the single egg-cell. 

Heiice we may say that the peculiar constitution of an animal 
does in part determine the length of time which must elapse before 
reproduction begins. The period before reproduction is however 
only part of the whole life of an animal, which of course extends 
over the total period during which the animal exists. 

Hitherto it has always been assumed that the duration of this 
total period is solely determined by the constitution of the ani- 
mal's body. But the assumption is erroneous. The strength of 



THE DURATION OF LIFE. 9 

the spring- which drives the wheel of life does not solely depend 
upon the size of the wheel itself or upon the material of which it 
is made ; and, leaving the metaphor, duration of life is not ex- 
clusively determined by the size of the animal, the complexity 
of its structure, and the rate of its metabolism. The facts are 
plainly and clearly opposed to such a supposition. 

How, for instance, can we explain from this point of view the 
fact that the queen -ant and the workers live for many years, while 
the males live for a few weeks at most ? The sexes are not dis- 
tinguished by any great difference in size or complexity of body, 
or in the rate of metabolism. In all these three particulars they 
must be looked upon as precisely the same, and yet there is this 
immense difference between the lengths of their lives. 

I shall return later on to this and other similar cases, and for 
the present I assume it to be proved that physiological con- 
siderations alone cannot determine the duration of life. It is not 
these which alone determine the strength of the spring which 
moves the machinery of life ; we know that springs of different 
strengths may be fixed in machines of the same kind and quality. 
This metaphor is however imperfect, because we cannot imagine 
the existence of any special force in an organism which deter- 
mines the duration of its life ; but it is nevertheless useful because 
( it emphasises the fact that the duration of life is forced upon 
the organism by causes outside itself, just as the spring is fixed in 
its place by forces outside the machine, and not only fixed in its 
place, but chosen of a certain strength so that it will run down 
after a certain time. 

To put it briefly, I consider that duration of life is really de- 
pendent upon adaptation to external conditions, that its length, 
whether longer or shorter, is governed by the needs of the species, 
and that it is determined by precisely the same mechanical process 
of regulation as that by which the structure and functions of an 
organism are adapted to its environment. 

Assuming for the moment that these conclusions are valid, let 
us ask how the duration of life of any given species can have 
been determined by their means. In the first place, in regulating- 
duration of life, the advantage to the species, and not to the 
individual, is alone of any importance. This must be obvious 
to any one who has once thoroughly thought out the process of 



10 THE DURATION OF LIFE. 

natural selection. It is of no importance to the species whether 
the individual lives longer or shorter, but it is of importance 
that the individual should be enabled to do its work towards the 
maintenance of the species. This work is reproduction, or the 
formation of a sufficient number of new individuals to compensate 
the species for those which die. As soon as the individual has 
performed its share in this work of compensation, it ceases to be 
of any value to the species, it has fulfilled its duty and may die. 
But the individual may be of advantage to the species for a longer 
period if it not only produces ofispring, but tends them for a 
longer or shorter time, either by protecting, feeding, or instructing 
them. This last duty is not only undertaken by man, but also 
by animals, although to a smaller extent ; for instance, birds teach 
their young to fly, and so on. 

We should therefore expect to find that, as a rule, life does not 
greatly outlast the period of reproduction except in those species 
which tend their young ; and as a matter of fact we find that this 
is the case. 

All mammals and birds outlive the period of reproduction, but 
this never occurs among insects except in those species which 
tend their yoving. Furthermore, the life of all the lower animals 
ceases also with the end of the reproductive period, as far as we 
can judge. 

Dm'ation of life is not however determined in this way, but 
only the point at which its termination occurs relatively to the 
cessation of reproduction. The duration itself depends first upon 
the length of time which is required for the animal to reach 
maturity— that is, the duration of its youth, and, secondly, upon 
the length of the period of fertility — that is the time which is 
necessary for the individual to produce a sufficient number of de- 
scendants to ensure the perpetuation of the species. It is precisely 
this latter point which is determined by external conditions. 

There is no species of animal which is not exposed to de- 
struction through various accidental agencies — by hunger or 
cold, by drought or flood, by epidemics, or by enemies, whether 
beasts of prey or parasites. We also know that these causes of 
death are only apparently accidental, or at least that they can 
only be called accidental as far as a single individual is concerned. 
As a matter of fact a far greater number of individuals perish 



THE DURATION OF LIFE. 11 

through the operation of these ag-encies than by natural death. 
There are thousands of species of which the existence depends upon 
the destruction of other species ; as, for example, the various kinds 
of fish which feed on the countless minute Crustacea inhabiting 
our lakes. 

It is easy to see that an individual is, ceteris paribus, more ex- 
posed to accidental death when the natural term of its life becomes 
longer ; and therefore the longer the time required by an in- 
dividual for the production of a sufficient number of descendants to 
ensure the existence of the species, the greater will be the number 
of individuals which perish accidentally before they have fulfilled 
this important duty. Hence it follows, first, that the number of 
dfescendants produced by any individual must be greater as the 
duration of its reproductive period becomes longer ; and, secondly, 
the surprising result that nature does not tend to secure the 
longest possible life to the adult individual, but, on the contrary, 
tends to shorten the period of reproductive activity as far as 
possible, and with this the duration of life ; but these conclusions 
only refer to the animal and not to the vegetable world. 

All this sounds very paradoxical, but the facts show that it is 
true. At first sight numerous instances of remarkably long life . 
seem to refute the argument, but the contradictions are only 
apparent and disappear on closer investigation. 

Birds as a rule live to a surprisingly great age. Even the 
smallest of our native singing birds lives for ten years, while the 
nightingale and blackbird live from twelve to eighteen years. 
A pair of eider ducks were observed to make their nest in the 
same place for twenty years, and it is believed that these birds 
sometimes reach the age of nearly one hundred years. A cuckoo, 
which was recognised by a peculiar note in its call, was heard in 
the same forest for thirty-two consecutive years. Birds of prey, 
and birds which live in marshy districts, become much older, for 
they outlive more than one generation of men. 

Schinz mentions a bearded vulture which was seen sitting on 
a rock upon a glacier near Grindelwald, and the oldest men in 
Grindelwald had, when boys, seen the same bird sitting on the 
same rock. A white-headed vulture in the Schonbrunn Zoo- 
logical Gardens had been in captivity for ii 8 years, and many 
examples are known of eagles and falcons reaching an age 



12 THE DURATION OF LIFE. 

of over loo years. Finally, we must not forget Humboldt's ^ 
Atur parrot from the Orinoco, concerning which the Indians said 
that it could not be understood because it spoke the language of 
an extinct tribe. 

It is therefore necessary to ask how far we can show that such 
long lives are really the shortest which are possible under the 
circumstances. 

Two factors must here be taken into consideration ; first, that 
the young of birds are greatly exposed to destructive agencies ; 
and, secondly, that the structure of a bird is adapted for flight and 
therefore excludes the possibility of any great degree of fertility. 

Many birds, like the stormy petrel, the diver, guillemot, and 
other sea-birds, lay only a single egg, and breed (as is usually the 
case with birds) only once a year. Others, such as birds of prey, 
pigeons, and humming-birds, lay two eggs, and it is only those 
which fly badly, such as jungle fowls and pheasants, which produce 
a number of eggs (about twenty), and the young of these very 
species are especially exposed to those dangers which more or less 
affect the offspring of all birds. Even the eggs of our most 
powerful native bird of prey, the golden eagle, which all animals 
fear, and of which the eyrie, perched on a rocky height, is beyond 
the reach of any enemies, are veiy frequently destroyed by late 
frosts or snow in spring, and, at the end of the year in winter, the 
young birds encounter the fiercest of foes, viz. hunger. In the 
majority of birds, the egg, as soon, as it is laid, becomes exposed to 
the attacks of enemies ; martens and weasels, cats and owls, buzzards 
and crows are all on the look out for it. At a later period the 
same enemies destroy numbers of the helpless young, and in winter 
many succumb in the struggle against cold and hunger, or to the 
numerous dangers which attend migration over land and sea, 
dangers which decimate the young birds. 

It is impossible directly to ascertain the exact number which 
are thus destroyed; but we can arrive at an estimate -by an 
indirect method. If we agree with Darwin and Wallace in 
believing that in most species a certain degree of constancy 
is maintained in the number of individuals of successive gene- 
rations, and that therefore the number of individuals within 
the same area remains tolerably uniform for a certain period of 
' Humboldt's 'Ansichten der Natur.' 



THE DURATION OF LIFE. 13' 

time ; it follows that, if we know the fertility and the average 
duration of life of a species, we can calculate the number of those 
which perish before reaching" maturity. Unfortunately the average 
length oriife is hardly known with certainty in the case of any 
species of bird. Let us however assume, for the sake of argument, 
that the individuals of a certain species live for ten years, and that 
they lay twenty eggs in each year ; then of the aoo eggs which 
are laid during the ten years, which constitute the lifetime of an 
individual, 198 must be destroyed, and only two will reach maturity, 
if the number of individuals in the species is to remain constant. 
Or to take a concrete example ; let us fix the duration of life in 
the golden eagle at 60 years, and its period of immaturity (of which 
the length is not exactly known) at ten years, and let us assume 
that it lays two eggs a year; — then a pair will produce 100 eggs 
in 50 years, and of these only two will develope into adult birds ; 
and thus on an average a pair of eagles will only succeed in bring- 
ing a pair of young to maturity once in fifty years. And so far 
from being an exaggeration, this calculation rather under-estimates 
the proportion of mortality among the young ; it is sufficient how- 
ever to enforce the fact that the number of young destroyed must 
reach in birds a very high figure as compared with the number of 
those which survive^. 

If this argument holds, and at the same time the fertility from 
physical and other grounds cannot be increased, it follows that 
a relatively long life is the only means by which the maintenance 
of the species of birds can be secured. Hence a great length 
of life is proved to be an absolute necessity for birds. 

I have already mentioned that these animals demonstrate most 
clearly that physiological considerations do not by any means suffice 
to explain the duration of life. Although all vital processes take 
place with greater rapidity and the temperature of the blood is 
higher in birds than in mammals, yet the former greatly surpass 
the latter in length of life. Only in the largest Mammalia, — the 
whales and the elephants — is the duration of life equal to or 
perhaps greater than that of the longest lived birds. If we com- 
pare the relative weights of these animals, the Mammalia are 
everywhere at a disadvantage. Even such large animals as the horse 
and bear only attain an age of fifty years at the outside ; the lion 

^ See Appendix, note i, p. 36. 



14 THE DURATION OF LIFE. 

lives about thirty-five years, the wild boar twenty-five, the sheep 
fifteen, the fox fourteen, the hare ten, the squirrel and the mouse six 
years ^; but the golden eagle, though it does not weigh more 
than from 9-1 3 pounds, and is thus intermediate as regards weight 
between the hare and the fox, attains nevertheless an age which is 
ten times as long. The explanation of this difference is to be found 
first in the much greater fertility of the smaller Mammalia, such 
as the rabbit or mouse, and secondly in the much lower mortality 
among the young of the larger Mammalia. The minimum duration 
of life necessary for the maintenance of the species is therefore 
much lower than it is among birds. Even here, however, we are 
not yet in possession of exact statistics indicating the number of 
young destroyed ; but it is obvious that Mammalia possess over 
birds a great advantage in their intra-uterine development. In 
Mammalia the destruction of young only begins after birth, while 
in birds it begins dming the development of the embryo. This 
distinction is in fe«t carried even further, for many mammals 
protect their young against enemies for a long time after birth. 

It is unnecessary to go further into the details of these cases, or 
to consider whether and to what extent every class of the animal 
kingdom conforms to these principles. Thus to consider all or 
even most of the classes of the animal kingdom would be quite 
impossible at the present time, because our knowledge of the 
duration of life among animals is very incomplete. Biological 
problems have for a long time excited less interest than morpho- 
logical ones. There is nothing or almost nothing to be found in 
existing zoological text books upon the duration of life in animals ; 
and even monographs upon single classes, such as the Amphibia, 
reptiles, or even birds, contain very little on this subject. When 
we come to the lower animals, knowledge on this point is almost 
entirely wanting. I have not been able to find a single reference 
to the age in Echinodermata, and very little about that of worms, 
Crustacea, and Coelenterata^. The length of life in many mol- 
luscan species is very well known, because the age can be deter- 
mined by markings on the shell ^ But even in this group, any 
exact knowledge, such as would be available for our purpose, is still 

' See Appendix, note 2, p. 38. 
" See Appendix, note 4, p. 54. 
^ See Appendix, note 5, p. 55. 



THE DURATION OF LIFE. 15 

wanting' concerning such necessary points as the degree of fertility, 
the relation to other animals, and many other factors. 

Data the most exact in all respects are found among the insects ^, 
and to this class I will for a short time direct your special atten- 
tion. We will first consider the dui'ation of larval life. This 
varies very greatly, and chiefly depends upon the nature of the 
food, and the ease or difiiculty with which it can be procured. The 
larvae of bees reach the pupal stage in five to six days ; but it is 
well known that they are fed with substances of high nutritive 
value (honey and pollen), and that they require no great effort to 
obtain the food, which lies heaped up around them. The larval 
life in many Ichneumoniclae is but little longer, being passed in 
a parasitic condition within other insects ; abundance of accessible 
food is thus supplied by the tissues and juices of the host. Again, 
the larvae of the blow-fly become pupae in eight to ten days, 
although they move actively in boring their way under the skin 
and into the tissues of the dead animals upon which they live. 
The life of the leaf-eating caterpillars of butterflies and moths lasts 
for six weeks or longer, corresponding to the lower nutritive value 
of their food and the greater expenditure of muscular energy in 
obtaining it. Those caterpillars which live upon wood, such as 
Cossus ligniperda^ have a larval life of two to three years, and the 
same is true of hymenopterous insects with similar habits, such as 
Sirex. 

Furthermore, predaceous larvae require a long period for attaining 
their full size, for they can only obtain their prey at rare intervals 
and by the expenditure of considerable energy. Thus among the 
dragon-flies larval life lasts for a year, and among many may-flies 
even two or three years. 

All these results can be easily understood from well-known physio- 
logical principles, and they indicate that the length of larval life is 
very elastic, and can be extended as circumstances demand ; for 
otherwise carnivorous and wood-eating larvae could not have sur- 
vived in the phyletic development of insects. Now it would be 
a great mistake to suppose that there is any reciprocal relation 
between duration of life in the larva and in the mature insect, 
or imago ; or, to put it differently, to suppose that the total 
duration of life is the same in insects of the same size and activity, 

^ See Appendix, note 3, p. 38. 



16 THE DURATION OP LIFE. 

80 that the time which is spent in the larval state is, as it were, 
deducted from the life of the imago, and vice versa. That this 
cannot be the case is shown by the fact already alluded to, that 
among- bees and ants larval life is of the same length in males and 
females, while there is a difference of some years between the lengths 
of their lives as imagos. 

The life of the imago is generally very short, and not only ends 
with the close of the period of reproduction, as was mentioned 
above, but this latter period is also itself extremely short ^. 

The larva of the cockchafer devours the roots of plants for a 
period of four years, but the mature insect with its more complex 
structure endures for a comparatively short time ; for the beetle itself 
dies in about a month after completing its metamorphosis. And 
this is by no means an extreme case. Most butterflies have an 
even shorter life, and among the moths there are many species (as 
in the Fsyclddae) which only live for a few days, while others 
again, which reproduce by the parthenogenetic method, only live for 
twenty-four hours. The shortest life is found in the imagos of 
certain may-flies, which only live four to five hours. They emerge 
from the pupa-case towards the evening, and as soon as their 
wings "have hardened, they begin to fly, and pair with one another. 
Then they hover over the water ; their eggs are extruded all at 
once, and death follows almost immediately. 

The short life of the imago in insects is easily explained by the 
principles set forth above. Insects belong to the number of those 
animals which, even in their mature state, are very liable to be 
destroyed by others which are dependent upon them for food ; but 
they are at the same time among the most fertile of animals, and 
often produce an astonishing number of eggs in a very short time. 
And no better arrangement for the maintenance of the species 
under such circumstances can be imagined than that supplied by 
diminishing the duration of life, and simultaneously increasing the 
rapidity of reproduction. 

This general tendency is developed to very different degrees 
according to conditions peculiar to each species. The shorten- 
ing of the period of reproduction, and the duration of life to the 
greatest extent which is possible, depends upon a number of co- 
operating circumstances, which it is impossible to enumerate 
' See Appendix, note 3, p. 38. 



THE DUKATION OF LIFE. 17 

completely. Even the manner in which the eg-gs are laid may- 
have an important effect. If the larva of the may-fly lived upon 
some rare and widely distributed food-plant instead of at the 
bottom of streams, the imagos would be compelled to live longer, 
for they would be obliged — Kke many moths and butterflies — to 
lay their eggs singly or in small clusters, over a large area. This 
would require both time and strength, and they could not retain 
the rudimentary mouth which they now possess, for they would 
have to feed in order to acquire sufiicient strength for long flights ; 
and — whether they were carnivorous like dragon-flies, or honey- 
eating like butterflies — their feeding would itself cause a further 
expenditure of both time and strength, which would necessitate a 
still fui-ther increase in the duration of life. And as a matter of 
fact we find that dragon-flies and swift-flying hawk-moths often 
live for six or eight weeks and sometimes longer. 

We must also remember that in many species the eggs are not 
mature immediately after the close of the pupal stage, but that 
they only gradually ripen during the life of the imago, and 
frequently, as in many beetles and butterflies, do not ripen simul- 
taneously, but only a certain number at a time. This depends, 
first, upon the amount of reserve nutriment accumulated in the body 
of the insect during larval life ; secondly, upon various but entirely 
different circumstances, such as the power of flight. Insects which 
fly swiftly and are continually on the wing, like hawk-moths and 
dragon-flies, cannot be burdened with a very large number of ripe 
eggs. In these cases the gradual ripening of the eggs becomes 
necessary, and involves an increase in the duration of life. In. 
Lepidoptera, we see how the power of flight diminishes step by 
step as soon as other circumstances permit, and simultaneously how 
the eggs ripen more and more rapidly, while the length of life 
becomes shorter, until a minimum is reached. Only two stages 
in the process of transformation can be mentioned here. 

The strongest flyers — the hawk-moths and butterflies — must be 
looked upon as the most specialised and highest types among the 
Lepidoptera. Not only do they possess organs for flight in their 
most perfect form, but also organs for feeding — the characteristic 
spiral proboscis or ' tongue.' 

There are certain moths (among the Bombyces) of which the 
males fly as well as the hawk-moths, while the females are unable 

c 



18 THE DURATION OF LIFE. 

to use their large wings for flight, because the body is too heavily 
weighted by a mass of eggs, all of which reach maturity at the same 
time. Such species, as for instance Aglia tan, are unable to dis- 
tribute their eggs over a wide area, but are obliged to lay them all 
in a single spot. They can however do this without harm to the 
species, because their caterpillars live upon forest trees, which pro- 
vide abundant food for a larger number of larvae than can be pro- 
duced by the eggs of a single female. The eggs of Aglia tan are 
deposited directly after pairing, and shortly afterwards the insect 
dies at the foot of the tree among the moss-covered roots of which 
it has passed the winter in the pupal state. The female moth seldom 
lives for more than three or four days ; but the males which fly 
swiftly in the forests, seeking for the less abundant females, live 
for a much longer period, certainly from eight to fourteen days ^. 

The females of the Psychidae also deposit all their eggs in one 
place. The grasses and Kchens upon which their caterpillars live 
grow close at hand upon the surface of the earth and stones, and 
hence the female moth does not leave the ground, and generally 
does not even quit the pupa-case, within which it lays its eggs ; 
as soon as this duty is finished, it dies. In relation to these habits 
the wings and mouth of the female are rudimentary, while the 
male possesses perfectly developed wings. 

The causes which have regulated the length of life in these cases 
are obvious enough, yet still more striking illustrations are to be 
found among insects which live in colonies. 

The duration of life varies with the sex in bees, wasps, ants, and 
Termites : the females have a long life, the males a short one ; and 
there can be no doubt that the explanation of this fact is to be found 
in adaptation to external conditions of life. 

The queen-bee — the only perfect female in the hive — lives two 
to three years, and often as long as five years, while the male bees 
or drones only live four to five months. Sir John Lubbock has 
succeeded in keeping female and working ants alive for seven 
years — a great age for insects ^, — while the males only lived a few 
weeks. 

* This estimate is derived from observation of the time during which these insects 
are to be seen upon the wing. Direct observations upon the duration of life in this 
species are unknown to me. 

[^ Sir John Lubbock has now kept a queen ant alive for nearly 15 years. See note 
» on p. 51.— E. B. P.] 



THE DURATION OF LIFE. 19 

These last examples become readily intelligible when we remember 
that the males neither collect food nor help in building- the hive. 
Their value to the colony ceases with the nuptial flight, and from 
the point of view of utility it is easy to understand why their lives 
should be so short ^. But the case is very diflPerent with the female. 
The longest period of reproduction possible, when accompanied by 
very great fertility, is, as a rule, advantageous for the mainten- 
ance of the species. It cannot however be attained in most 
insects, for the capability of living long would be injurious if all 
individuals fell a prey to their enemies before they had completed 
the full period of life. Here it is otherwise : when the queen-bee 
returns from her nuptial flight, she remains within the hive until 
her death, and never leaves it. There she is almost completely 
secure from enemies and from dangers of all kinds ; thousands of 
workers armed with stings protect, feed, and warm her ; and in 
short there is every chance of her living through the full period of 
a life of normal length. And the case is entirely similar* with the 
female ant. In neither of these insects is there any reason why 
the advantages which follow from a lengthened period of repro- 
ductive activity should be abandoned ^. 

That an increase in the length of life has actually taken place in 
such cases seems to be indicated by the fact that both sexes of the 
saw-flies — the probable ancestors of bees and ants — have but a 
short life. On the other hand, the may-flies afford an undoubted 
instance of the shortening of life. Only in certain species is life as 
short as I have indicated above ; in the majority it lasts for one or 
more days. The extreme cases, with a life of only a few hom's, 
form the end of a line of development tending in the direction of a 
shortened life. This is made clear by the fact that one of these 
may-flies {Falingenia) does not even leave its pupa-skin, but repro- 
duces in the so-called sub-imago stage. 

It is therefore obvious that the duration of life is extremely 
variable, and not only depends upon physiological considerations, 
but also upon the external conditions of life. With every change 
in the structure of a species, and with the acquisition of new 
habits, the length of its life may, and in most cases must, be 
altered. 

* See Appendix, notes 7 and 9, pp. 59 and 63. 
^ See Appendix, note 6, p. 58. 
C 2 



20 THE DURATION OF LIFE. 

In answering the question as to the means bj which the lengthen- 
ing or shortening of life is brought about, our first appeal must 
be to the process of natural selection. Duration of life, like every 
other characteristic of an organism, is subject to individual fluc- 
tuations. From our experience with the human species we know 
that long life is hereditary. As soon as the long-lived individuals 
in a species obtain some advantage in the struggle for existence, 
they will gradually become dominant, and those with the shortest 
lives will be exterminated. 

So far everything is quite simple ; but hitherto we have only 
considered the external mechanism, and we must now further in- 
quire as to the concomitant internal means by which such processes 
are rendered possible. 

This brings us face to face with one of the most diflBcult problems 
in the whole range of physiology, — the question of the origin of 
death. As soon as we thoroughly understand the circumstances 
upon which normal death depends in general, we shall be able to 
make a further inquiry as to the circumstances which influence its 
earlier or later appearance, as well as to any functional changes in 
the organism which may produce such a result. 

The changes in the organism which result in normal death, — 
senility so-called, — have been most accurately studied among men. 
We know that with advancing age certain alterations take place 
in the tissues, by which their functional activity is diminished ; that 
these changes gradually increase, and finally either lead to direct or 
so-called normal death, or produce indirect death by rendering the 
organism incapable of resisting injuries due to external influences. 
These senile changes have been so well described from the time of 
Burdach and Bichat to that of Kussmaul, and are so well known, 
that I need not enter into further details here. 

In answer to an inquiry as to the causes which induce these 
changes in the tissues, I can only suggest that the cells which 
form the vital constituents of tissues are worn out by prolonged 
use and activity. It is conceivable that the cells might be thus 
worn out in two ways ; either the cells of a tissue remain the 
same throughout life, or else they are being continually replaced 
by younger generations of cells, which are themselves cast off" in 
their turn. 

In the present state of our knowledge the former alternative can 



THE DURATION OF LIFE. 21 

hardly be maintained. Millions of blood corpuscles are continually 
dying and being- rej)laced by new ones. On both the internal and 
external surfaces of the body countless epithelial cells are being 
incessantly removed, while new ones arise in their place ; the activity 
of many and probably of all glands is accompanied by a change in 
their cells, for their secretions consist partly of detached and partly 
of dissolved cells ; it is stated that even the cells of bone, connective 
tissue, and muscle undergo the same changes, and nervous tissue 
alone remains, in which it is doubtful whether such a renewal of 
cells takes place. And yet as regards even this tissue, certain facts 
are known which indicate a normal, though probably a slow renewal 
of the histological elements. I believe that one might reasonably 
defend the statement, — in fact, it has already found advocates, — 
that the vital processes of the higher (i.e. multicellular) animals 
are accompanied by a renewal of the morphological elements in 
most tissues. 

This statement leads us to seek the origin of death, not in the 
waste of single cells, but in the limitation of their powers of repro- 
duction. Death takes place because a worn-out tissue cannot for 
ever renew itself, and because a capacity for increase by means of 
cell-division is not everlasting, but finite ^. This does not however 
imply that the immediate cause of death lies in the imperfect re- 
newal of cells, for death would in all cases occur long before the 
reproductive power of the cells had been completely exhausted. 
Functional disturbances will appear as soon as the rate at which the 
worn-out cells are renewed becomes slow and insufficient. 

But it must not be forgotten that death is not always preceded 
by senility, or a period of old age. For instance, in many of 
the lower animals death immediately follows the most important 
deed of the organism, viz. reproduction. Many Lepidoptera, all 
may-flies, and many other insects die of exhaustion immediately 
after depositing their eggs. Men have been known to die from 
the shock of a strong passion. Sulla i^ said to have died as 
the result of rage, whilst Leo X succumbed to an excess of joy. 
Here the psychical shock caused too intense an excitement of the 
nervous system. In the same manner the exercise of intense effort 
may also produce a similarly fatal excitement in the above- 
mentioned insects. At any rate it is certain that when, for some 
* See Appendix, note 8, p. 59. 



22 THE DURATION OF LIFE. 

reason, this effort is not made, the insect lives for a somewhat 
longer period. 

It is clear that in such animals as insects we can only speak 
figuratively of normal death, if we mean by this an end which is 
not due to accident. In these animals an accidental end is the rule, 
and is therefore, strictly speaking, normal ^. 

Assuming the truth of the above-mentioned hypothesis as to the 
causes of normal death, it follows that the number of cell-genera- 
tions which can proceed from the egg-cell is fixed for every 
species, at least within certain limits ; and this number of cell- 
generations, if attained, corresponds to the maximum duration of 
life in the individuals of the species concerned. Shortening of life 
in any species must depend upon a decrease in the number of 
successive cell-generations, while conversely, the lengthening of 
life depends upon an increase in the number of cell-generations over 
those which were previously possible. 

Such changes actually take place in plants. When an annual 
plant becomes perennial, the change — one in every way possible 
— can only happen by the production of new shoots, i. e. by an 
increase in the number of cell-generations. The process is not so 
obvious in animals, because in them the formation of young cells 
does not lead to the production of new and visible parts, for the 
new material is merely deposited in the place of that which is worn 
out and disappears. Among plants, on the other hand, the old 
material persists, its cells become lignified, and it is built over by 
new cells which assume the functions of life. 

It is certainly true that the question as to the necessity of death 
in general does not seem much clearer from this point of view than 
from the purely physiological one. This is because we do not know 
why a cell must divide 10,000 or 100,000 times and then suddenly 
stop. It must be admitted that w' e can see no reason why the 
power of cell-multiplication should not be unlimited, and why the 
organism should not therefore be endowed with everlasting life. 
In the same manner, from a physiological point of view, we might 
admit that we can see no reason why the functions of the organism 
should ever cease. 

It is only from the point of view of utility that we can imder- 



* See Appendix, note 9, p. 63. 



THE DUKATION OF LIFE. 23 

stand the necessity of death. The same arguments which were 
employed to explain the necessity for as short a life as possible, will 
with but slight modification serve to explain the common necessity 
of death ^. 

Let us imagine that one of the higher animals became immortal ; 
it then becomes perfectly obvious that it would cease to be of 
value to the species to which it belonged. Suppose that such an 
immortal individual could escape all fatal accidents, through infinite 

[^ After reading these proofs Dr. A. E. Wallace kindly sent me an unpublished 
note upon the production of death by means of natural selection, written by him 
some time between 1865 and 1870. The note contains some ideas on the subject, 
which were jotted down for further elaboration, and were then forgotten until 
recalled by the argument of this Essay. The note is of great interest in relation to 
Dr. Weismann's suggestions, and with Dr. Wallace's permission I print it in full 
below. 

'The Action of Natural Selection in Producing Old Age, 
Decay, and Death. 
' Supposing organisms ever existed that had not the power of natural reproduc- 
tion, then since the absorptive surface would only increase as the square of the 
dimensions while the bulk to be nourished and renewed would increase as the cube, 
there must soon arrive a limit of growth. Now if such an organism did not produce 
its like, accidental destruction would put an end to the species. Any oi-ganism 
therefore that, by accidental or spontaneous fission, could become two organisms, 
and thus multiply itself indefinitely without increasing in size beyond the limits 
most favourable for nourishment and existence, could not be thus exterminated : 
since the individual only could be accidentally destroyed, — the race would survive. 
But if individuals did not die they would soon multiply inordinately and would inter- 
fere with each other's healthy existence. Food would become scarce, and hence the 
larger individuals would probably decompose or diminish in size. The deficiency of 
nourishment would lead to parts of the organism not being renewed ; they would 
become fixed, and liable to more or less slow decomposition as dead parts within a 
living body. The smaller organisms would have a better chance of finding food, the 
larger ones less chance. That one which gave off several small portions to form 
each a new organism would have a better chance of leaving descendants like 
itself than one which divided equally or gave off a large part of itself. Hence it 
would happen that those which gave off very small portions would probably soon 
after cease to maintain their own existence while they would leave a numerous 
offspring. This state of things would be in any case for the advantage of the race, 
and would therefore, by natural selection, soon become established as the regular 
course of things, and thus we have the origin of old age, decay, and death ; for it is 
• evident that when one or more individuals have provided a sufficient number of 
successors they themselves, as consumers of nourishment in a constantly increasing 
degree, are an injury to those successors. Natural selection therefore weeds them 
out, and in many cases favours such races as die almost immediately after they have 
left successors. Many moths and other insects are in this condition, living only to 
propagate their kind and then immediately dying, some not even taking any food 
in the perfect and reproductive state.' — E. B. P.] 



24 THE DUKATION OF LIFE. 

time, — a supposition which is of course hardly conceivable. The 
individual would nevertheless be unable to avoid, from time to 
time, slight injuries to one or another part of its body. The 
injured parts could not regain their former integrity, and thus the 
longer the individual lived, the more defective and crippled it 
would become, and the less perfectly would it fulfil the purpose of 
its species. Individuals are injui'ed by the operation of external 
forces, and for this reason alone it is necessary that new and perfect 
individuals should continually arise and take their place, and this 
necessity would remain even if the individuals possessed the power 
of living eternally. 

From this follows, on the one hand, the necessity of reproduction, 
and, on the other, the utility of death. Worn-out individuals are 
not only valueless to the species, but they are even harmful, for 
they take the place of those which are sound. Hence by the 
operation of natural selection, the life of our hypothetically im- 
mortal individual would be shortened by the amount which was 
useless to the species. It would be reduced to a length which 
would afford the most favourable conditions for the existence of as 
large a number as possible of vigorous individuals, at the same 
time. 

If by these considerations death is shown to be a beneficial 
occurrence, it by no means follows that it is to be solely accounted 
for on grounds of utility. Death might also depend upon causes 
which lie in the nature of life itself. The floating of ice upon 
water seems to us to be a useful arrangement, although the fact 
that it does float depends upon its molecular structure and not 
upon the fact that its doing so is of any advantage to us. In like 
manner the necessity of death has been hitherto explained as due to 
causes which are inherent in organic nature, and not to the fact 
that it may be advantageous. 

I do not however believe in the validity of this explanation ; 
I consider that death is not a primary necessity, but that it has 
been secondarily acquired as an adaptation. I believe that life is 
endowed with a fixed duration, not because it is contrary to its 
nature to be unlimited, but because the unlimited existence of 
individuals would be a luxury without any corresponding advantage. 
The above-mentioned hypothesis upon the origin and necessity of 
death leads me to believe that the organism did not finally cease 



THE DURATION OP LIFE. 25 

to renew the worn-out cell material because the nature of the cells 
did not permit them to multiply indefinitely, but because the power 
of multiplying indefinitely was lost when it ceased to be of use. 

I consider that this view, if not exactly proved, can at any rate 
be rendered extremely probable. 

It is useless to object that man (or any of the higher animals) 
dies from the physical necessity of his nature, just as the specific 
gravity of ice results from its physical nature. I am quite ready to 
admit that this is the case, John Hunter, supported by his ex- 
periments on anabiosis, hoped to prolong the life of man indefinitely 
by alternate freezing and thawing ; and the Veronese Colonel 
Aless. Guaguino made his contemporaries believe that a race 
of men existed in Russia, of which the individuals died regularly 
every year on the 27th of November, and returned to life on 
the 34th of the following April. There cannot however be the 
least doubt, that the higher organisms, as they are now con- 
structed, contain within themselves the germs of death. The 
. question however arises as to how this has come to pass ; and 
I reply that death is to be looked upon as an occurrence which 
is advantageous to the species as a concession to the outer con- 
ditions of life, and not as an absolute necessity, essentially inherent 
in life itself. 

Death, that is the end of life, is by no means, as is usually 
assumed, an attribute of all organisms. An immense number of 
low organisms do not die, although they are easily destroyed, being 
killed by heat, poisons, &c. As long, however, as those conditions 
which are necessary for their life are fulfilled, they continue to live, 
and they thus carry the potentiality of unending life in them- 
selves. I am speaking not only of the Amoebae and the low 
unicellular Algae, but also of far more highly organized unicellular 
animals, such as the Infusoria. 

The process of fission in the Amoeba has been recently much 
discussed, and I am well aware that the life of the individual is 
generally believed to come to an end with the division which gives 
rise to two new individuals, as if death and reproduction were the 
same thing. But this process cannot be truly called death. Where 
is the dead body ? what is it that dies ? Nothing dies ; the body 
of the animal only divides into two similar parts, possessing the 
same constitution. Each of these parts is exactly like its parent, 



26 THE DUBATION OF LIFE. 

lives in the same manner, and finally also divides into two halves. 
As far as these organisms are concerned, death can only be spoken 
of in the most figurative sense. 

There are no grounds for the assumption that the two halves of 
an Amoeba are differently constituted internally, so that after a 
time one of them will die while the other continues to live. Such 
an idea is disproved by a recently discovered fact. It has been 
noticed in Engli/pha (one of the Foraminifera) and in other low 
animals of the same group, that when division is almost complete, 
and the two halves are only connected by a short strand, the proto- 
plasm of both parts begins to circulate, and for some time passes 
backwards and forwards between the two halves. A complete 
mingling of the whole substance of the animal and a resulting 
identity in the constitution of each half is thus brought about 
before the final separation ^. 

The objection might perhaps be raised that, if the parent animal 
does not exactly die, it nevertheless disappears as an individual. I 
cannot however let this pass unless it is also maintained that the 
man of to-day is no longer the same individual as the boy of twenty 
years ago. In the growth of man, neither structiire nor the com- 
ponents of structure remain precisely the same ; the material is 
continually changing. If we can imagine an Amoeba endowed 
■with self-consciousness, it might think before dividing ' I will give 
birth to a daughter,' and I have no doubt that each half would 
regard the other as the daughter, and would consider itself to be 
the original parent. We cannot however appeal to this criterion of 
personality in the Amoeba, but there is nevertheless a criterion 
which seems to me to decide the matter : I refer to th6 continuity 
of life in the same form. 

Now if numerous organisms, endowed with the potentiality of 
never-ending life, have real existence, the question arises as to 
whether the fact can be understood from the point of view of 
utility. If death has been shown to be a necessary adaptation for 
the higher organisms, why should it not be so for the lower also ? 
Are they not decimated by enemies ? are they not often imperfect ? 
are they not worn out by contact with the external world? 
Although they are certainly destroyed by other animals, there is 

* See Appendix, note lo, p. 64. 



THE DURATION OF LIFE. 27 

nothing- comparable to that deterioration of the body which takes 
place in the higher organisms. Unicellular animals are too simply 
constructed for this to be possible. If an infusorian is injured by 
the loss of some part of its body, it may often recover its former 
integrity, but if the injury is too great it dies. The alternative is 
always perfect integrity or complete destruQtion. 

We may now leave this part of the subject, for it is obvious that 
normal death, that is to say, death which arises from internal 
causes, is an impossibility among" these lower organisms. In those 
species at any rate in which fission is accompanied by a circulation 
of the protoplasm of the parent, the two halves must possess the 
same qualities. Since one of them is endowed with a potentiality 
for unending- life, and must be so endowed if the species is to persist, 
it is clear that the other exactly similar half must be endowed 
with equal potentiality. 

Let us now consider how it happened that the multicellular 
animals and plants, which arose from unicellular forms of life, came 
to lose this power of living for ever. 

The answer to this question is closely bound up with the principle 
of division of labour which appeared among multicellular organisms 
at a very early stage, and which has gradually led to the production 
of greater and greater complexity in their structure. 

The first multicellular organism was probably a cluster of similar 
cells, but these units soon lost their original homogeneity. As the 
result of mere relative position, some of the cells were especially 
fitted to provide for the nutrition of the colony, while others 
undertook the work of reproduction. Hence the single group 
would come to be divided into two groups of cells, which may 
be called somatic and reproductive — the cells of the body as op- 
posed to those which are concerned with reproduction. This 
differentiation was not at first absolute, and indeed it is not always 
so to-day. Among the lower Metazoa, such as the polypes, the 
capacity for reproduction still exists to such a degree in the somatic 
cells, that a small number of them are able to give rise to a new 
organism, — in fact new individuals are normally produced by means 
of so-called buds. Furthermore, it is well known that many of the 
higher animals have retained considerable powers of regeneration ; 
the salamander can replace its lost tail or foot, and the snail can 
reproduce its horns, eyes, etc. 



28 THE DUKATION OF LIFE. 

As the complexity of tte Metazoan body increased, the two 
groups of cells became more sharply separated from each other. 
Very soon the somatic cells surpassed the reproductive in number, 
and during- this increase they became more and more broken up 
by the principle of the division of labour into sharply separated 
systems of tissues. As these changes took place, the power of 
reproducing large parts of the organism was lost, while the power 
of reproducing the whole individual became concentrated in the 
reproductive cells alone. 

But it does not therefore follow that the somatic cells were 
compelled to lose the power of unlimited cell-production, although 
in accordance with the law of heredity, they could only give 
rise to cells which resembled themselves, and belonged to the same 
differentiated histological system. But as the fact of normal 
death seems to teach us that they have lost even this power, the 
causes of the loss must be sought outside the organism, that is 
to say, in the external conditions of life ; and we have already 
seen that death can be very well explained as a secondarily ac- 
quired adaptation. The reproductive cells cannot lose the capacity 
for unlimited reproduction, or the species to which they belong 
would suffer extinction. But the somatic cells have lost this 
power to a gradually increasing extent, so that at length they 
became restricted to a fixed, though perhaps very large number of 
cell-generations. This restriction, which implies the continual influx 
of new individuals, has been explained above as a result of the 
impossibility of entirely protecting the individual from accidents, 
and from the deterioration which follows them. Normal death 
could not take place among unicellular organisms, because the indi- 
vidual and the reproductive cell are one and the same : on the 
other hand, normal death is possible, and as we see, has made its 
appearance, among multicellular organisms in which the somatic 
and reproductive cells are distinct. 

I have endeavoured to explain death as the result of restriction 
in the powers of reproduction possessed by the somatic cells, and I 
have suggested that such restriction may conceivably follow from a 
limitation in the number of cell -generations possible for the cells 
of each organ and tissue. I am unable to indicate the molecular 
and chemical properties of the cell upon which the duration of 
its power of reproduction depends : to ask this is to demand an 



THE DURATION OF LIFE. 29 

explanation of the nature of heredity — a problem the solution of 
which may still occupy many generations of scientists. At present 
we can hardly venture to propose any explanation of the real nature 
of heredity. 

But the question must be answered as to whether the kind and 
degree of reproductive power resides in the nature of the cell itself, 
or in any way depends upon the quality of its nutriment. 

Virchow, in his ' Cellular Pathology,' has remarked that the cells 
are not only nourished, but that they actively supply themselves 
with food. If therefore the internal condition of the cell decides 
whether it shall accept or reject the nutriment which is offered, it 
becomes conceivable that all cells may possess the power of refusing 
to absorb nutriment, and therefore of ceasing to undergo further 
division. 

■^ Modern embryology affords us many proofs, in the segmentation 
of the ovum, and in the subsequent developmental changes, that 
the causes of the different forms of reproductive activity witnessed 
in cells lie in the essential nature of the cells themselves. Why 
does the segmentation of one half of certain eggs proceed twice as 
rapidly as that of the other half? why do the cells of the ectoderm 
divide so much more quickly than those of the endoderm ? Why 
does not only the rate, but also the number of cells produced (so 
far as we can follow them) always remain the same ? Why does 
the multiplication of cells in every part of the blastoderm take 
place with the exact amount of energy and rapidity necessary to 
produce the various elevations, folds, invaginations, etc., in which 
the different organs and tissues have their origin, and from which 
finally the organism itself arises ? There can be no doubt that 
the causes of all these phenomena lie within the cells them- 
selves ; that in the ovum and the cells which are immediately 
derived from it, there exists a tendency towards a certain determined 
(I might almost say specific) mode and energy of cell-multiplica- 
tion. And why should we regard this inherited tendency as con- 
fined to the building up of the embryo ? why should it not also 
exist in the young, and later in the mature animal ? The pheno- 
mena of heredity which make their appearance even in old age 
afford us proofs that a tendency towards a certain mode of cell- 
multiplication continues to regulate the growth of the organism 
during- the whole of its life. 



30 THE DURATION OP LIFE. 

The above-mentioned considerations show us that the degree 
of reproductive activity present in the tissues is regulated by 
internal causes while the natural death of an organism is the 
termination — the hereditary limitation — of the process of cell- 
division, which began in the segmentation of the ovum. 

Allow me to suggest a further consideration which may be com- 
pared with the former. The org-anism is not only limited in time, 
but also in space : it not only lives for a limited period, but it can 
only attain a limited size. Many animals grow to their full size 
long before their natural end : and although many fishes, reptiles, and 
lower animals are said to grow during the whole of their life, we do 
not mean by this that they possess the power of unlimited growth 
any more than that of unlimited life. There is everywhere a 
maximum size, which, as far as our experience goes, is never sur- 
passed. The mosquito never reaches the size of an elephant, nor 
the elephant that of a whale. 

Upon what does this depend ? Is there any external obstacle 
to growth ? Or is the limitation entirely imposed from within ? 

Perhaps you may answer, that there is an established relation 
between the increase of surface and mass, and it cannot be denied 
that these relations do largely determine the size of the body. 
A beetle could never reach the size of an elephant, because, co'i- 
stituted as it is, it would be incapable of existence if it attair jd 
such dimensions. But nevertheless the relations between sui-f ice 
and mass do not form the only reason why any given individ lal 
does not exceed the average size of its species. Each individ lal 
does not strive to grow to the largest possible size, until the 
absorption from its digestive area becomes insufficient for its mass ; 
but it ceases to grow because its cells cannot be sufiiciently nourished 
in consequence of its increased size. The giants which occasionally 
appear in the human species prove that the plan upon which man 
is constructed can also be carried out on a scale which is far larger 
than the normal one. If the size of the body chiefly depends upon 
amount of nutriment, it would be possible to make giants and 
dwarfs at will. But we know, on the contrary, that the size of 
the body is hereditary in families to a very marked extent ; in fact 
so much so that the size of an individual depends chiefly upon 
heredity, and not upon amount of food, 
r These observations point to the conclusion that the size of the 



THE DURATION OP LIFE. 31 

individual is in reality pre-determined, and that it is potentially 
contained in the egg from which the individual developes. 

We know further that the growth of the individual depends 
chiefly upon the multiplication of cells and only to a slight extent 
upon the growth of single cells. It is therefore clear that a 
limit of growth is imposed by a limitation in the processes by 
which cells are increased, both as regards the number of cells 
produced and the rate at which they are formed. How could we 
otherwise explain the fact that an animal ceases to grow long 
before it has reached the physiologically attainable maximum of its 
species, without at the same time suffering any loss of vital 
energy ? 

In many cases at least, the most important duty of an organism, 
viz. reproduction, follows upon the attainment of full size — a fact 
which induced Johannes Miiller to reject the prevailing hypothesis 
which explained the death of animals as due to ' the influences 
of the inorganic environment, which gradually wear away the life 
of the individual.' He argued that, if this were the case, ' the 
organic energy of an individual would steadily decrease from the 
beginning,' while the facts indicate that this is not so ^. 

If it is further asked why the egg should give rise to a fixed 
ni,'^ber of cell-generations, although perhaps a number which 
va-"jes widely within certain limits, we may now refer to the opera- 
tion of natural selection upon the relation of surface to mass, and 
up%n other physiological necessities which are peculiar to the species. 
Because a certain size is the most favourable for a certain plan 
of organization, the process of natural selection determined that 
such a size should be within certain variable limits, characteristic 
of each species. This size is then transmitted from generation to 
generation, for when once established as normal for the species, the 
most favourable size is potentially present in the reproductive cell 
from which each individual is developed. 

If this conclusion holds, and I believe that no essential objection 
can be raised against it, then we have in the limitation in space 
a process which is exactly analogous to the limitation in time, 
which we have already considered. The latter limitation — the 
duration of life — also depends upon the multiplication of cells, the 

* Johannes Miiller, ' Physiologie,' Bd. T. p. 31, Berlin, 1840. 



32 THE DURATION OF LIFE. 

rapid increase of which first gave rise to the characteristic form of 
the mature body, and then continued at a slower rate. In the 
mature animal, cell- reproduction still goes on, but it no longer 
exceeds the waste ; for some time it just compensates for loss, and 
then begins to decline. The waste is not compensated for, the 
tissues perform their functions incompletely, and thus the way for 
death is prepared, until its final appearance by one of the three 
great Atria mortis. 

I admit that facts are still wanting upon which to base this 
hypothesis. It is a pure supposition that senile changes are due to 
a deficient reproduction of cells : at the same time this supposition 
gains in probability when we are enabled to reduce the limitations 
of the organism in both time and space to one and the same 
principle. It cannot however be asserted under any circumstances 
that it is a pure supposition that the ovum possesses a capacity 
for cell-multiplication which is limited both as to numbers produced 
and rate of production. The fact that each species maintains an 
average size is a sufficient proof of the truth of this conclusion. 

Hitherto I have only spoken of animals and have hardly men- 
tioned plants. I should not have been able to consider them at 
all, had it not happened that a work of Hildebrand's ^ has recently 
appeared, which has, for the first time, provided us with exact 
observations on the duration of plant-life. 

The chief results obtained by this author agree very well with 
the view which I have brought before you to-day. Hildebrand 
shows that the duration of life in plants also is by no means 
completely fixed, and that it may be very considerably altered 
through the agency of the external conditions of life. He shows 
that, in course of time, and under changed conditions of life, an 
annual plant may become perennial, or vice versa. The external 
fectors which influence the duration of life are here however essen- 
tially different, as indeed we expect them to be, when we remember 
the very different conditions under which the animal and vegetable 
kingdoms exist. During the life of animals the destruction of 
mature individuals plays a most important part, but the existence 
of the mature plant is fairly well secured ; their chief period of 
destruction is during youth, and this fact has a direct influence 

^ See Appendix, note 12, p. 65. 



THE DURATION OP LIFE. 83 

upon the degi'ee of fertility, but not upon the duration of life. 
Climatic considerations, especially the periodical changes of summer 
and winter, or wet and dry seasons, are here of greater importance. 

It must then be admitted that the dependence of the duration of 
life upon the external conditions of existence is alike common to 
plants and animals. In both kingdoms the high multicellular 
forms with well-dijQferentiated organs contain the germs of death, 
while the low unicellular organisms are potentially immortal. 
Furthermore, an undying succession of reproductive cells is possessed 
by all the higher forms, although this may be but poor consolation to 
the conscious individual which perishes. Johannes Miiller is there- 
fore right, when in the sentence quoted at the beginning of my 
lecture, he speaks of an ' appearance of immortality ' which passes 
from each individual into that which succeeds it. That which 
remains over, that which persists, is not the individual itself, — 
not the complex aggregate of cells which is conscious of itself, — 
but an individuality which is outside its consciousness, and of a low 
order, — an individuality which is made up of a single cell, which 
arises from the conscious individual. I might here conclude, but 
I wish first, in a few words, to protect myself against a possible 
misunderstanding. 

I have repeatedly spoken of immortality, first of the unicellular 
organism, and secondly of the reproductive cell. By this word 
I have merely intended to imply a duration of time which appears 
to be endless to our human faculties. I have no wish to enter into 
the question of the cosmic or telluric origin of life on the earth. 
An answer to this question will at once decide whether the power 
of reproduction possessed by these cells is in reality eternal or only 
immensely prolonged, for that which is without beginning is, 
and must be, without end. 

The supposition of a cosmic origin of life can only assist us 
if by its means we can altogether dispense with any theory of 
spontaneous generation. The mere shifting of the origin of life 
to some other far-off world cannot in any way help us. A truly 
cosmic origin in its widest significance will rigidly limit us to 
the statement — omne vivum e vivo — to the idea that life can only 
arise from life, and has always so arisen, — to the conclusion that 
organic beings are eternal like matter itself. 

Experience cannot help us to decide this question ; we do not 

D 



84 THE DURATION OF LIFE. 

know whether spontaneous generation was the commencement of 
life on the earth, nor have we any direct evidence for the idea 
that the process of development of the living world carries the 
end within itself, or for the converse idea that the end can only 
be brought about by means of some external force. 

I admit that spontaneous generation, in spite of all vain efforts 
to demonstrate it, remains for me a logical necessity. We cannot 
regard organic and inorganic matter as independent of each other 
and both eternal, for organic matter is continually passing, without 
residuum, into the inorganic. If the eternal and indestructible are 
alone without beginning, then the non-eternal and destructible must 
have had a beginning. But the organic world is certainly not 
eternal and indestructible in that absolute sense in which we 
apply these terms to matter itself. We can, indeed, kill all organic 
beings and thus render them inorganic at will. But these changes 
are not the same as those which we induce in a piece of chalk 
by pouring sulphuric acid upon it ; in this case we only change 
the form, and the inorganic matter remains. But when we pour 
sulphuric acid upon a worm, or when we burn an oak tree, these 
organisms are not changed into some other animal and tree, but 
they disappear entirely as organized beings and are resolved into 
inorganic elements. But that which can be completely resolved 
into inorganic matter must have also arisen from it, and must 
owe its ultimate foundation to it. The organic might be con- 
sidered eternal if we could only destroy its form, but not its nature. 

It therefore follows that the organic world must once have arisen, 
and further that it will at some time come to an end. Hence we 
must speak of the eternal duration of unicellular organisms and 
of reproductive cells in the Metazoa and Metaphyta in that par- 
ticular sense which signifies, when measured by our standards, an 
immensely long time. 

Yet who can maintain that he has discovered the right answer to 
this important question? And even though the discovery were 
made, can any one believe that by its means the problem of life 
would be solved ? If it were established that spontaneous genera- 
tion did actually occur, a new question at once arises as to the 
conditions under which the occurrence became possible. How can 
we conceive that dead inorganic matter could have come together 
in such a manner as to form living protoplasm, that wonderful 



THE DURATION OF LIFE. 35 

and complex substance which absorbs foreign material and changes 
it into its own substance, in other words grows and multiplies ? 

And so, in discussing this question of life and death, we come at 
last — as in all provinces of human research — upon problems which 
appear to us to be, at least for the present, insoluble. In fact it 
is the quest after perfected truth, not its possession, that falls to 
our lot, that gladdens us, fills up the measure of our life, nay! 
hallows it. 



D a 



APPENDIX. 

Note I. The Duration of Life among Birds. 

There is less exact knowledge upon this subject than we might 
expect, considering the existing number of ornithologists and 
ornithological societies with their numerous publications. It has 
neither" been possible nor necessary for my purpose to look up all 
the widely-scattered references which are to be found upon the 
subject. Many of these are doubtless unknown to me ; for we are 
still in want of a compilation of accurately determined observations 
in this department of zoology. I print the few facts which I have 
been able to collect, as a slight contribution towards such a com- 
pilation. 

Small singing birds live from eight to eighteen years : the 
nightingale, in captivity, eight years, but longer according to 
some writers : the blackbird, in captivity, twelve years, but both 
these birds live longer in the natural state. A ' half-bred nightin- 
gale built its nest for nine consecutive years in the same garden ' 
(Naumann, ' Vogel Deutschlands,' p. 'j6). 

Canary birds in captivity attain an age of twelve to fifteen 
years (1, c, p. 76). 

Ravens have lived for almost a hundred years in captivity 
(1. c, Bd. I. p. 125). 

Magpies in captivity live twenty years, and, ' without doubt,' 
much longer in the natural state (1. c, p. 346). 

Parrots ' in captivity have reached upwards of a hundred years ' 
(Lc, p. 125). 

A single instance of the cuckoo (alluded to in the text) is men- 
tioned by Naumann as reaching the age of thirty-two years (1. c, 
p. 76). 

Fowls live ten to twenty years, the golden pheasant fifteen years, 
the turkey sixteen years, and the pigeon ten years (Oken, ' Natur- 
geschichte, Vogel,' p. 387). 



APPENDIX. 37 

A golden eagle whicli ' died at Vienna in the year 171 9, had been 
captured 104 years previously' (Brehm, ' Leben der Vogel,' p. 72). 

A falcon (species not mentioned) is said to have attained an age 
of 162 years (Knauer, ^Der Naturhistoriker,' Vienna, 1880). 

A white-headed vulture which was taken in 1706 died in the 
Zoological Gardens at Vienna (Schonbrunn) in 1824, thus living 
118 years in captivity (1. c). 

The example of the bearded vulture, mentioned in the text, is 
quoted from Schinz's 'Vogel der Schweiz,' p. 196. 

The wild goose must live for upwards of 100 years, according to 
Naumann (1. c, p. 127)- The proof of this is not, however, forth- 
coming. A wild goose which had been wounded reached its 
eighteenth year in captivity. 

Swans are said to have lived 300 years (?), (Naumann, I.e., p. 127). 

It is evident that observations upon the duration of life in wild 
birds can only rarely be made, and that they are usually the result 
of chance and cannot be verified. It is on this account all the 
more to be desired that every ascertained fact should be collected. 

If the long life of birds has been correctly interpreted as com- 
pensation for theii' feeble fertility and for the great mortality of 
their young, it will be possible to estimate the length of life in a 
species, without direct observation, if we only know its fertility and 
the percentage of individuals destroyed. This percentage can, how- 
ever, at best, be known only as an average. If we consider, for 
example, the enormous number of sea birds which breed in summer 
on the rocks and cliffs of the northern seas, and if we remember that 
the majority of these birds lay but one, or at most two eggs yearly, 
and that their young are exposed to very many destructive agencies, 
we are forced to the conclusion that they must possess a very long 
life, so that the breeding period may be many times repeated. 
Their number does not diminish. Year after year countless num- 
bers of these birds cover the rocks, from summit to sea line ; 
millions of them rest there, and rise in the air like a thick cloud 
whenever they are disturbed. Even in those localities which are 
every year visited by man in order to effect their capture, the 
number does not appear to decrease, unless the birds are disturbed 
and are therefore prompted to seek other breeding-places. From 
the small island of St. Kilda, off Scotland, 20,000 young gannets 
[Sula) and an immense number of eggs are annually collected ; 



88 THE DURATION OF LIFE. 

and although this bird only lays a single egg yearly and takes 
four years to attain maturity, the numbers do not diminish^. 
30,000 sea-gulls' eggs and 20,000 terns' eggs are yearly exported 
from the breeding-places on the island of Sylt, but in this case 
it appears that a systematic disturbance of the birds is avoided 
by the collectors, and no decrease in their numbers has yet taken 
place ^. The destruction of northern birds is not only caused by 
man, but also by various predaceous mammals and birds. Indeed 
the dense mass of birds which throng the cliffs is a cause of 
destruction to many of the young and to the eggs, which are 
pushed over the edge of the rocks. According to Brehm the foot 
of these cliffs is ' always covered with blood and the dead bodies of 
fledglings.' 

Such birds must attain a great age or they would have been 
exterminated long ago : the minimum duration of life necessary for 
the maintenance of the species must in their case be a very 
high one. 

Note 2. The Duration of Life among Mammals. 

The statements upon this subject in the text are taken from 
many sources ; from Giebel's ' Saugethiere,' from Oken's ' Natur- 
geschichte,' from Brehm's ' Illustrirtem Thierleben,' and from an 
essay of Knauer in the * Naturhistoriker,' Vienna, 1880. 

Note 3. The Duration of Life among Mature Insects. 

A short statement of the best established facts which I have been 
able to find is given below. I have omitted the lengthening of 
imaginal life which is due to hybernation in certain species. In 
almost all orders of insects there are certain species which emerge 
from the pupa in the autumn, but which first reproduce in the 
following spring. The time spent in the torpid condition during 
winter cannot of course be reckoned with the active life of the 
species, for its vital activity is either entirely suspended for a time by 
freezing [Anabiosis : Preyer ^), or it is at any rate never more than 
a vita minima, with a reduction of assimilation to its lowest point. 

* Oken, ' Naturgeschichte,' Stuttgart, 1837, Bd. IV. Abth. i. 
" Brehm, * Leben der Vogel,' p. 278. 

' ' Naturwissenschaftliche Thatsachen und Probleme,' Populiire Vortrage, Berlin, 
1880; ft(Ze Appendix. 



APPENDIX. ' 39 

The following account does not make any claim to contain all or 
even most of the facts scattered throug-h the enormous mass of 
entomological literature, and much less all that is privately; known 
by individual entomologists. It must therefore be looked upon as 
merely a first attempt, a nucleus, around which the principal facts 
can be gradually collected. It is ^nneeessaiy to give any special 
information as to the duration of larval life, for numerous and exact 
observations upon this part of the subject are contained in all ento- 
mological works. 

I. Orthoptera. 

Gryllotalpa. The eggs are laid in June or July, and the young 
are hatched in from two to three weeks ; they live through the 
winter, and become sexually mature in the following May or June. 
' When the female has deposited her eggs, her body collapses, and 
afterwards she does not survive much longer than a month.' 
' According as the females ai-e younger or older, they live a longer 
or shorter life, and hence some females are even found in the 
autumn' (Rosel, ' Insektenbelustigungen,' Bd. II. p. 92). Eosel 
believes that the female watches the eggs until they are hatched, 
and this explains the fact that she outlives the process of ovi- 
position by about a month. It is not stated whether the males die 
at an earlier period, 

Gryllus cawpestris becomes sexually mature in May, and sings from 
June till October, ' when they all die ' (Oken, ' Naturgeschichte,' 
Bd. II. Abth. iii. p. 1527). It is hardly probable that any single 
individual lives for the whole summer ; probably, as in the case of 
Gryllotalpa^ the end of the life of those individuals which first 
become mature, overlaps the beginning of the life of others which 
reach maturity at a later date. 

Lociida viriclissima and L. verrucivora are mature at the end of 
August ; they lay their eggs in the earth dliring the first half 
of September and then die. It is probable that the females do 
not live for more than four weeks in the mature state. It is not 
known whether the males of this or other species of locusts live for 
a shorter period. 

I have found Locusta cantana in plenty, from the beginning of 
September to the end of the month. In captivity they die after 
depositing their eggs : the males are probably more short-lived, for 



40 THE DURATION OF LIFE. 

towards the middle and end of September they are much less 
plentiful than the females. 

Acriclium migratoruim 'dies after the egg-s are laid' (Oken, 
' Naturgeschichte '). 

The male Termes probably live for a short time only, although 
exact observations upon the point are wanting. The females ' seem 
sometimes to live four or five years/ as I gather from a letter from 
Dr. Hagen, of Cambridge, Mass., U.S.A. 

Hjphemeridae. Rosel, speaking of Ephemera vulgata (' Insekten- 
belustigungen,' Bd. II. der Wasserinsekten, 2*® Klasse, p. 60 et seq.), 
says: — 'Their flight commences at sunset, and comes to an end before 
midnight, when the dew begins to fall.' ' The pairing generally 
takes place at night and lasts but a short time. As soon as the in- 
sects have shed their last skin, in the afternoon or evening, they fly 
about in thousands, and pair almost immediately ; but by the next 
day they are all dead. They continue to emerge for many days, so 
that when yesterday's swarm is dead, to-day a new swarm is seen 
emerging from the water towards the evening.' ' They not only drop 
their eggs in the water, but w^herever they may happen to be, — on 
trees, bushes, or the earth. Birds, trout and other fish lie in wait 
for them.' 

Dr. Hagen writes to me — 'It is only in certain species that 
life is so short. The female Palingenia does not live long enough 
to complete the last moult of the sub-imago. I believe that a 
female imago has never been seen. The male imago, often half in its 
sub-imago skin, fertilizes the female sub-imago and immediately 
the contents of both ovaries are extruded, and the insect dies. It 
is quite possible that the eggs pass out by rupturing the abdominal 
segments.' 

Lihellula. All dragon-flies live in the imago condition for some 
weeks ; at first they are not capable of reproduction, but after a few 
days they pair. 

Lepisma saccharina. An individual lived for two years in a pill- 
box, without any food except perhaps a little Lycopodium dust^. 

«< 
II. Neueoptera. 

Phri/ganifh ' live in the imago stage for at least a week and prob- 
ably longer, apparently without taking food ' (letter from Dr. Hagen). 
* ' Entomolog. Mag.,' vol, i. p. 537, 1833. 



APPENDIX. 41 

According" to the latest researches Thryganea grarulis'^ never con- 
tains food in its alimentary canal, but only air, although it contains 
the latter in such quantities that the anterior end of the chylific 
ventricle is dilated by it. 

III. Stuepsiptera. 

The larva requires for its development a rather shorter time than 
that which is necessary for the grub of the bee into the body of which 
it has bored. The pupa stage lasts eight to ten days. The male, 
which flies about in a most impetuous manner, lives only two to 
three hours, while the female lives for some days. Possibly the 
pairing does not take place until the female is two io three days old. 
The viviparous female seems to produce young only once in a life- 
time, and then dies : it is at present uncertain whether she also pro- 
duces young parthenogenetically (cf Siebold, ' Ueber Paedogenesis 
der Strepsipteren, ' Zeitschr. f. Wissensch. Zool., Band. XX, 1870). 

IV. Hemiptera. 

Aphis. Bonnet (' Observations sur les Pucerons,' Paris, 1745) had 
a parthenogenetic female of Aphis euonymi in his possession for 
thirty-one days, from its birth, during which time it brought forth 
ninety-five larvae. Gleichen kept a parthenogenetic female of 
Aphis mall fifteen to twenty-three days. 

Aphis foliornm ulmi. The mother of a colony which leaves 
the eg^ in May is 2'" long at the end of July : it therefore lives 
for at least two and a half months (De Geer, ' Abhandlungen zur 
Geschichte_ der Insekten,' 1783, III. p. ^'^). 

Phylloxera vastatrix. The males are merely ephemeral sexual 
organisms, they have no proboscis and no alimentary canal, and 
die immediately after fertilizing the female. 

Pemphigus terebinthi. The male as well as the female sexual in- 
dividuals are wingless and without a proboscis ; they cannot take 
food and consequently live but a short time, — far shorter than the 
parthenogenetic females of the same species (Derbes, ' Note sur les 
aphides du pistachier terebinthe,' Ann. des sci. nat., Tom. XVII, 
1872). 

Cicada. In spite of the numerous and laborious descriptions of 

* Imhof, ' Beitrage zur Anatomie der Ferla maxima,^ Inaug. Diss., Aarau, 1881. 



42 THE DURATION OP LIFE. 

the Cicadas which have appeared during* the last two centuries, I 
can only find precise statements as to the duration of life in the 
mature insect in a single species. P. Kalm, writing upon the 
North American Cicada septemdecim, which sometimes appears in 
countless numbers, states that ' six weeks after (such a swarm had 
been first seen) they had all disappeared.' Hildreth puts the life of 
the female at from twenty to twenty-five days. This agrees with 
the fact that the Cicada lays many hundred eggs (Hildreth states a 
thousand) ; sixteen to twenty at a time being inserted into a hole 
which is bored in wood, so that the female takes some time to lay 
her eggs (Oken, 'Naturgeschichte,' 2*®' Bd. 3*® Abth. p. 1588 et seq.). 
Acanthia lectularia. No observations have been made upon the 
bed bug from which the normal length of its life can be ascer- 
tained, but many statements tend to show that it is exceedingly 
long-lived, and this is advantageous for a parasite of which the food 
(and consequently growth and reproduction) is extremely precarious. 
They can endure starvation for an astonishingly long period, and 
can survive the most intense cold. Leunis (' Zoologie,' p. 659) 
mentions the case of a female which was shut up in a box and or- 
gotten : after six months' starvation it was found not only alive 
but surrounded by a circle of lively young ones. Goze found 
bugs in the hangings of an old bed which had not been used for 
six years : * they appeared white like paper.' I have myself ob- 
served a similar case, in which the starving animals were quite 
transparent. De Geer placed some bugs in an unheated room in 
the cold winter of 1772, when the thermometer feU to — 33°C: 
they passed the whole winter in a state of torpidity, but revived 
in the following May. (De Geer, Bd. III. p. 165, and Oken, 
' Naturgeschichte,' 2*"' Bd. 3*^ Abth. p. 1613.) 

V. DiPTERA. 

Pulex irritans. Oken says of the flea (' Naturgeschichte,' Bd. II. 
Abth. 2, p. 759) that ' death follows the deposition of the eggs in 
the course of two or three days, even if the opportunity of sucking 
blood is given them.' The length of time which intervenes between 
the emergence from the cocoon and fertilization or the deposition 
of eggs is not stated. 

SarcopTiaga carnaria. The female fly dies ten to twelve hours 
after the birth of the viviparous larvae ; the time intervening 



APPENDIX. 43 

between the exit from the cocoon and the birth of the young is 
not given (Oken, quoting Reaumur, ' Mem. p. s. a I'hist. Insectes,' 
Paris, 1740-48, IV). 

Musca domestica. In the summer the common house-fly begins 
to lay eggs eight days after leaving the cocoon : she then lays 
several times. (See Gleichen, ' Geschichte der gemeinen Stuben- 
fliege,' Nm'emberg, 1764.) 

Eristalis tenax. The larva of this large fly lives in liquid 
manure, and has been described and figured by Reaumiir as the rat- 
tailed larva. I kept a female which had just emerged from the 
cocoon, from August 30th till October 4th, in a large gauze-covered 
glass vessel. The insect soon learnt to move freely about in its 
prison, without attempting to escape ; it flew round in cii'cles, with 
a characteristic buzzing sound, and obtained abundant nourish- 
ment from a solution of sugar, provided for it. From September 
lath it ceased to fly about, except when frightened, when it would 
fly a little way ofi". I thought that it was about to die, but 
matters took an unexpected turn, and on the 26th of September it 
laid a large packet of eggs, and again on the 29th of the same 
month another packet of similar size. The flight of the animal 
had been probably impeded by the weight of the mass of ripe eggs 
in its body. The deposition of eggs was probably considerably 
retarded in this case, because fertilization had not taken place. 
The fly died on the 4th of October, having thus lived for thirty-five 
days. Unfortunately, I have been unable to make any experiments 
as to the duration of life in the female when males are also present. 

VI. Lepidopteea. 

I am especially indebted to Mr. W. H. Edwards ^, of Coalburgh, 
W. Virginia, and to Dr. Speyer, of Rhoden, for valuable letters 
relating to this order. 

The latter writes, speaking of the duration of life in imagos 
generally : — ' It is, to my mind, improbable that any butterfly can 
live as an imago for a twelvemonth. Specimens which have lived 
through the winter are only rarely seen in August, even when the 
summer is late. A worn specimen of Vanessa cardui has, for 

^ Mr. Edwards has meanwhile published these communications in full ; cf. ' On 
the length of life of Butterflies,' Canadian Entomologist, 1881, p. 205. 



44 THE DURATION OP LIFE. 

instance, been found at this time ' (' Entomolog. Nachrichten,' 
i88i, p. 146). 

In answer to my question as to whether the fact that certain 
Lepidoptera take no solid or liquid food, and are, in fact, without 
a functional mouth, may be considered as evidence for an adapta- 
tion of the length of life to the rapid deposition of eggs, Dr. Speyer 
replies : — ' The wingless females of the Ps^c/iidae do not seem to 
possess a mouth, at any rate I cannot find one in Tsyclie 'unlcolor 
{graminella). They do not leave the case during life, and certainly 
do not drink water. The same is true of the wingless female of 
Heterogynis, and of Orgym ericae, and probably of all the females of 
the genus Orgyia ; and as far as I can judge from cabinet specimens, 
it is probably true of the males of Heterogynis and PsycJie. I have 
never seen the day-flying Saturnidae, Bombycidae, and other Lepi- 
doptera with a rudimentary proboscis, settle in damp places, or 
suck any moist substance, and I doubt if they would ever do this. 
The sucking apparatus is probably deficient.' 

In answer to my question as to whether the males of any species 
of butterfly or moth are known to pass a life of diflferent length 
from that of the female, Dr. Speyer stated that he knew of no ob- 
servations on this point. 

The following are the only instances of well-established direct 
observations upon single individuals, in my possession ^ : — 

Pieris napi, var. hryoniae $ and ? , captured on the wing : lived 
in confinement ten days, and were then killed. 

Vanessa prorsa lived at most ten days in confinement. 

Vanessa urticae lived ten to thirteen days in confinement. 

Papilio ajax. According to a letter from Mr. W. H. Edwards, 
the female, when she leaves the pupa, contains unripe eggs in her 
body, and lives for about six weeks — calculating from the first 
appearance of this butterfly to the disappearance of the same 
generation 2. The males live longer, and continue to fly when very 
worn and exhausted. A worn female is very seldom seen ; — ' I 
believe the female does not live long after laying her eggs, but 
this takes some days, and probably two weeks.' 

Lycaena violacea. According to Mr. Edwards, the first brood of 
this species lives three to four weeks at the most. 

• When no authority is given, the observations are my own. 

' In the paper quoted above, Edwards, after weighing all the evidence, reduces 
the length of life from three to four weeks. 



APPENDIX. 45 

Smerinthus tiliae. A female, which had just emerged from the 
pupa, was caught on June 24th ; on the 29th pairing took place ; 
on the 1st of July she laid about eighty eggs, and died the following 
day. She lived nine days, taking no food during this period, and 
she only survived the deposition of eggs by a single day. 

Macroglossa stellatarum. A female, captured on the wing and 
already fertilized, lived in confinement from June 28th to July 4th. 
During this time she laid about eighty eggs, at intervals and 
singly; she then disappeared, and must have died, although the 
body could not be found among the grass at the bottom of the 
cage in which she was confined. 

Saturnia pyri. A pair which quitted the cocoons on the 24th or 
25th of April, remained in coitu from the 26th until May 2nd — 
six or seven days ; the female then laid a number of eggs, and died. 

Psyche graminella. The fertilized female lives some days, and 
the unfertilized female over a week (Speyer). 

Solenobia triqueirella. 'The parthenogenetic form (I refer to 
the one which I have shown to be parthenogenetic in Oken's ' Isis,' 
1846, p. 30) lays a mass of eggs in the abandoned case, soon after 
emergence. The oviposition causes her body to shrivel up, and 
some hours afterwards she dies. The non-parthenogenetic female 
of the same species remains for many days, waiting to be fertilized ; 
if this does not occur, she lives over a week.' ' The parthenogenetic 
female lives for hardly a day, and the same is true of the partheno- 
genetic females of another species of Solenobia ' {8. inconspicuella ?). 
Letter from Dr. Speyer. 

Psyche calcella, 0. The males live a very short time ; ' those 
which leave the cocoon in the evening are found dead on the 
following morning, with their wings fallen ofi", at the bottom of 
their cage.' Dr. Speyer. 

Eiqnthecia, sp. [Geometridae), 'when well-fed, live for three to four 
weeks in confinement ; the males fertilize the females frequently, 
and the latter continue to lay eggs when they are very feeble, and 
are incapable of creeping or flying.' Dr. Speyer. 

The conclusions and speculations in the text seem to be suffi- 
ciently supported from this short series of observations. There 
remains, as we see, much to be done in this field, and it would 
well repay a lepidopterist to undertake some exact observations 
upon the length of life in difierent butterflies and moths, with 



46 THE DUBATION OP LIFE. 

reference to the conditions of life — the mode of egg-laying", the 
degeneracy of the wings, and of the external mouth-parts or the 
closure of the mouth itself. It would be well to ascertain whether 
such closure does really take place, as it undoubtedly does in certain 
plant-lice. 

VII. COLEOPTERA. 

Melolontha vulgaris. Cockchafers, which I kept in an airy cage 
with fresh food and abundant moisture, did not in any case live 
longer than thirty-nine days. One female only, out of a total 
number of forty-nine, lived for this period ; a second lived thirty- 
six days, a third thirty-five,, and a fourth and fifth twenty-four 
days ; all the rest died earlier. Of the males, only one lived as 
long as twenty-nine days. These periods are less by some days 
than the true maximum duration of life, for the beetles were cap- 
tui-ed in the field, and had lived for at least a day ; but the differ- 
ence cannot be great, when we remember that out of forty-nine 
beetles, only three females lived thirty-five to thirty-nine days, and 
only one male twenty-nine days. Those that died earlier had 
probably lived for some considerable time before being caught. 

Exact experiments with pupae which have survived the winter 
would show whether the female really lives for ten days more than 
the male, or whether the results of my experiment were merely 
accidental. I may add that coitus frequently took place daring 
the period of captivity. One pair, observed in this condition on 
the 1 7th, separa'ted in the evening ; they paired again on the 
morning of the i8th, and sepai'ated in the middle of the day. 
Coitus took place between another pair on the a and, and again on 
the 26th. 

I watched the gradual approach of death in many individuals : 
some days before it ensued, the insects became sluggish, ceased to 
fly and to eat, and only crept a little way off when disturbed : they 
then fell to the ground and remained motionless, apparently dead, 
but moved their legs when irritated, and sometimes automatically. 
Death came on gradually and imperceptibly; from time to time 
there was a slow movement of the legs, and at last, after some 
hours, all signs of life ceased. 

In One case only I found bacteria present in great numbers in 
the blood and tissues ; in the other individuals which had recently 



APPENDIX. At 

died, the only noticeable change was the unusual dryness of the 
tissues. 

Carabus auratus. An experiment with an individual, caug-ht on 
May 37th, gave the length of life at fourteen days ; this is 
probably below the average, since the beetles are found, in the wild 
state, from the end of May until the beginning of July. 

Lucanus cervits. Captured individuals, kept in confinement, and 
fed on a solution of sugar, never lived longer than fourteen days, 
and as a rule not so long. The beetles appear in June and July, 
and certainly cannot live much over a month. As is the case with 
many beetles appearing dm-ing certain months, the length of the 
individual life is shorter than the period over which they are found. 
Accurate information, especially as to any difference between the 
lengths of life in the sexes, is not obtainable. 

Isolated accounts of remarkably long lives among beetles are to 
be found scattered throughout the literature of the subject. Dr. 
Hagen, of Cambridge, Mass., has been kind enough to draw my 
attention to these, and to send me some observations of his own. 

Ceramhyx Iteros. One individual lived in confinement from 
August until the following year ^. 

Saperda carcharias. An individual lived from the 5th of July 
until the 34th of July of the next year ^. 

Btiprestis splendens. A living individual was removed from a 
desk which had stood in a London counting-house for thirty years ; 
from the condition of the wood it was evident that the larva had 
been in it before the desk was made^. 

Bhjps mortisaga. One individual lived three months, and two 
others three years. 

Blaps fatidica. One individual which was left in a box and for- 
gotten, was found alive when the box was opened six years after- 
wards. 

Blaps obtusa. One lived a year and a half in confinement. 

Eleodes grandis and E. dentipes. Eight of these beetles from 
California were kept in confinement and without food for two years 
by Dr. Gissler, of Brooklyn ; they were then sent to Dr. Hagen 
who kept them another year. 

Goliathus cacicus. One individual lived in a hot-house for five 

months. 

^ 'Entomolog. Mag.,' vol. i. p. 527, 1823. 



48 THE DURATION OF LIFE. 

In addition to these cases, Dr. Hagen writes to me: 'Among- the 
beetles which live for more than a year, — Blaps, Pasimachus, {Cara- 
hidae) — and among ants, almost thirty per cent, are found with the 
cuticle worn out and cracked, and the powerful mandibles so greatly 
worn down that species were formerly founded upon this point. 
The mandibles are sometimes worn down to the hypodermis.' 

From the data before me I am inclined to beUeve that in certain 
beetles the normal length of life extends over some years, and this 
is especially the case with the Blajndae. It seems probable that in 
these cases another factor is present, — a vita minima, or apparent 
death, a sinking of the vital processes to a minimum in consequence 
of starvation, which we might call the hunger sleep, after the ana- 
logy of winter sleep. The winter sleep is usually ascribed to cold 
alone, and some insects certainly become so torjDid that they appear 
to be dead when the temperature is low. But cold does not affect 
all insects in this way. Among bees, for example, the activity of 
the insects diminishes to a marked extent at the beginning of 
winter, but if the temperature continues to fall, they become active 
again, run about, and as the bee-keepers say, * try to warm them- 
selves by exercise ' ; by this means they keep some life in them. 
If the frost is very severe, they die. In the tropics the period of 
hibernation for many animals coincides with the time of maximum 
heat and drought. This shows that the organism can be brought 
into the condition of a vita minima in various ways, and it would 
not be at all remarkable if such a state were induced in certain in- 
sects by hunger. Exact experiments however are the only means 
by which such a suggestion can be tested, and I have already com- 
menced a series of experiments. The fact that certain beetles live 
without food for many years (even six) can hardly be explained on 
any other supposition, for these insects consume a fair amount of 
food under normal conditions, and it is inconceivable that they 
could live for years without food, if the metabolism were carried on 
with its usual energy. 

A very striking example, showing that longevity may be induced 
by the • lengthening of the period of reproductive activity, is com- 
municated to me by Dr. Adler in the following note : ' Three years 
ago I accidentally noticed that ovoviviparous development takes 
place in Chrysomela varians, — a fact which I afterwards discovered 
had been already described by another entomologist. 



APPENDIX. 49 

'The egg" passes through all the developmental stages in the 
ovary; when these are completed the egg is laid, and a minute or 
two afterwards the larva breaks through the egg-shell. In each 
division of the ovary the eggs undergo development one at a time ; 
it therefore follows that they are laid at considerable intervals, so 
that a long life becomes necessary in order to ensure the develop- 
ment of a sufficiently long series of eggs. Hence it comes about 
that the females live a full year. Among other species of Chryso- 
mela two generations succeed each other in a year, and the duration 
of life in the individual varies from a few months to half a year.' 

VIII. Hymenoptera. 

Cynipidae. I have been unable to find any accurate accounts of 
the duration of life in the imagos of saw-flies or ichneumons ; but 
on the other hand I owe to the kindness of Dr. Adler, an excellent 
observer of the Cynipidae, the precise accounts of that family which 
are in my possession. I asked Dr. Adler the general question as to 
whether there was any variation in the duration of life among the 
Cynipidae corresponding to the conditions under which the deposi- 
tion of eggs took place ; whether those species which lay many 
eggs, or of which the oviposition is laborious and protracted, lived 
longer than those species which lay relatively few eggs, or easily and 
quickly find the suitable places in which to deposit them. 

Dr. Adler fully confirmed my suppositions and supported them 
by the following statements : — 

* The summer generation of Neurolerus [Spatheg aster) has the 
shortest life of all Cynipidae. Whether captured or reared from the 
galls I have only kept them alive on an average for three to four 
days. In this generation the work of oviposition requires the 
shortest time and the least expenditure of energy, for the eggs are 
simply laid on the surface of a leaf The number of eggs in the 
ovary is also smaller than that of other species, averaging about 
200. This form of Cynips can easily lay 100 eggs a day. 

' The summer generation of Bryophanta [Spat/iegaster Taschenlergi, 
verrucostis, etc.) lives somewhat longer ; I have kept them in con- 
finement for six to eight days. The oviposition requires a consider- 
able expenditure of time and strength, for the ovipositor has to 
pierce the rather tough mid-rib or vein of a leaf. The number of 
eggs in the ovary averages 300 to 400. 

E 



50 THE DUEATION OF LIFE. 

' The summer generation oi Andricus, which belongs to the exten- 
sive g-enus Aphilotrix, have also a long life. I have kept the smaller 
Andricus (such as A. midus, A. cirratus, A. noduli) alive for a week, 
and the larger [A. injlator, A. curvator^ A. ramuli) for two weeks. 
The smaller species pierce the young buds when quite soft, but the 
larger ones bore through the fully grown buds protected by tough 
scales. The ovary of the former contains 400 to 500 eggs, that of 
the latter over 600. 

'The agamic winter generations live much longer. The species of 
Neuroterus have the shortest life ; they live for two weeks at the 
outside ; on the other hand^ species of Aphilotrix live quite four 
weeks, and Bryophanta and Biorhiza even longer. I have kept 
Bryojihanta scutellaris alive for three months. The number of eggs 
in these agamic Cynipidae is much larger : Bryophanta and Aphilotrix 
contain 1200 and Neuroterus about 1000.' 

It is evidently, therefore, a general rule that the duration of life 
is directly proportional to the number of eggs and to the time and 
energy expended in oviposition. It must of course be understood 
that, here as in all other instances, these are not the only factors 
which determine the duration of life, but many other factors, at 
present unknown, may be in combination with them and assist in 
producing the result. For example, it is very probable that the 
time of year at which the images appear exerts some indirect 
influence. The long-lived Biorhiza emerges from the gall in the 
middle of winter, and at once begins to deposit eggs in the oak 
buds. Although the insect is not sensitive to low temperature, for 
I have myself seen oviposition proceeding when the thermometer 
stood at 5° R., yet very severe frost would certainly lead to inter- 
ruption and would cause the insect to shelter itself among dead 
leaves on the ground. Such interruptions may be of long duration 
and frequently repeated, so that the remarkably long life of this 
species may perhaps be looked upon as an adaptation to its winter 
life. 

Ants. Lasiusfiavus lays its eggs in the autumn, and the young 
larvae pass the winter in the nest. The males and females leave 
the cocoons in June, and pair during July and August. The males 
fly out of the nest with the females, but they do not return to it ; 
* they die shortly after pairing.' It is also believed that the females 
do not return to the nest, but found new colonies ; this point is 



APPENDIX. 51 

however one of the most uncertain in the natural history of ants. 
On the other hand it is quite certain that the female may live for 
years within the nest, continuing- to lay fertilized eggs. Old 
females are sometimes found in the colony, with their jaws worn 
down to the hypodermis. 

Breeding experiments confirm these statements. P. Huber ^ and 
Christ have already put the life of the female at three to four years, 
and Sir John Lubbock, who has been lately occupied with the 
natural history of ants, was able to keep a female worker of Formica 
sanguinea alive for five years ; and he has been kind enough to write 
and inform me that two females oi Formica fusca, which he captured 
in a wood together with ten workers, in December 1874, are still 
alive (July 1881), so that these insects live as images for six and a 
half years or more ^. 

* 'Eecherches sur les moeurs des Fourmis indigenes,' Genfeve, 1810. 

^ These two female ants were still alive on the 25th of September following Sir 
John Lubbock's letter, so that they live at least seven years. Cf. ' Observations on 
Ants, Bees, and Wasps,' Part VIII. p. 385 ; Linn. Soc. Joum. Zool., vol. xv. 1881. 

[Sir John Lubbock has kindly given me further information upon the duration of life 
of these two queen ants. Since the receipt of his letter, the facts have been published 
in the Journal of the Linnean Society (Zoology), vol. xx. p. 133. I quote in full 
the passage which refers to these ants : — 

' Longevity. — It may be remembered that my nests have enabled me to keep ants 
under observation for long periods, and that I have identified workers of Lasius niger 
and Formica fusca which were at least seven years old, and two queens of Formica 
fusca which have lived with me ever since December 1874. One of these queens, 
after ailing for some days, died on the 30th July, 1887. She must then have been 
more than thirteen years old. I was at first afraid that the other one might be 
aflFected by the death of her companion. She lived, however, until the 8th August, 
1888, when she must have been nearly fifteen years old, and is therefore by far the 
oldest insect on record. 

' Moreover, what is very extraordinary, she continued to lay fertile eggs. Thia 
remarkable fact is most interesting from a physiological point of view. Fertilization 
took place in 1874 at the latest. There has been no male in the nest since then, 
and, moreover, it is, I believe, well established that queen ants and queen bees are 
fertilized once for all. Hence the spermatozoa of 1874 must have retained their life 
and energy for thirteen years, a fact, I believe, unparalleled in physiology.' 
******* 

' I had another queen of Formica fusca which lived to be thirteen years old, and 
I have now a queen of Lasius niger which is more than nine years old, and still laya 
fertile eggs, which produce female ants.' 

Both the above-mentioned queens may have been considerably older, for it is im- 
possible to estimate their age at the time of capture. It is only certain (as Sir John 
Lubbock informs me in his letter) that ' they must have been at least nine months 
old (when captured), as the eggs oi F. fusca are laid in March or early in April.' 
The queens became gradually ' somewhat lethargic and stiff in their movements 

E 2 



52 THE DUBATION OF LIFE. 

On the other hand, Sir John Lubbock never succeeded in keeping 
the males 'alive lon^^er than a few weeks.' Both the older and 
more recent observers agree in stating that female ants, like 
queen bees, are always protected as completely as possible from 
injury and danger. Dr. A. Forel, whose thorough knowledge of 
Swiss ants is well known, writes to me, — * The female ants are only 
once fertilized, and are then tended by the workers, being cleaned 
and fed in the middle of the nest : one often finds them with only 
three legs, and with their chitinous armour greatly worn. They 
never leave the centre of the nest, and their only duty is to lay 
eggs.' 

With regard to the workers, Forel believes that their constitution 
would enable them to live as long as the females (as the experiments 
of Lubbock also indicate), and the fact that in the wild state they 
generally die sooner than the females is ' certainly connected with 
the fact that they are exposed to far greater dangers.' The same 
relation seems also to obtain among bees, but with them it has not 
been shown that in confinement the workers live as long as the 
queens. 

Bees. According to von Berlepsch ^ the queen may as an excep- 
tion live for five years, but as a rule survives only two or three 
years. The workers always seem to live for a much shorter period, 
generally less than a year. Direct experiments upon isolated or 
confined bees, or upon marked individuals in the wild state, do not 
prove this, but the statistics obtained by bee-keepers confirm the 
above. Every winter the numbers in a hive diminish from 
1 3,000-20,000 to 2000-3000. The queen lays the largest number 
of eggs in the spring, and the workers which die before the winter 
are replaced by those which emerge in the summer, autumn or 
during a mild winter. The queen lays eggs at such a variable 
rate throughout the year that the above-mentioned inequality in 
numbers is explained. The workers do not often live for more than 
six to seven months, and at the time of their greatest labour, (May 
to July), only three months. An attempt to calculate the length 
of life of the workers and drones by taking stock at the end of 

(before their death), but there was no loss of any limb nor any abrasion.' This last 
observation seems to indicate that queen ants may live for a much longer period in 
the wild state, for it is stated above that the chitin is often greatly worn, and some 
of the limbs lost (see pp. 48, 51, and 52). — E. B. P.] 

' A. von Berlepsch, 'Die Biene und ihre Zucht,' etc., 3rd ed.; Mannheim, 187a. 



APPENDIX. 53 

summer, gives six months for the former and four months for the 
latter ^. 

The drones do not as a rule live so long- as four months, for they 
meet with a violent death before the end of this period. The well- 
known slaughter of the drones is not, according to the latest obser- 
vations, brought about directly by means of the stings of the 
workers, but by these latter driving away the useless drones from 
the food so that they perish of starvation. 

Wasps. It is interesting that among these near relations of the 
bees, the life of the female should be much shorter, corresponding 
to the much lower degree of specialization found in the colonies. 
The females of Polistes gallica and of Vesjm not only lay eggs but 
take part in building the cells and in collecting food ; they are 
therefore obliged to use all parts of the body more actively and 
especially the wings, and are exposed to greater danger from 
enemies. 

It is well known from Leuckart's observations, that the so-called 
' workers ' of Polistes gallica and Bomhus are not arrested females 
like the workers of a bee-hive, but are females which although 
certainly smaller, are in every way capable of being fertilized and 
of reproduction. Von Siebold has nevertheless proved that they 
are not fertilized, but reproduce parthenogenetically. 

The fertilized female which survives the winter, commences to 
found a colony at the beginning of May : the larvae, which hatch 
from the first eggs, which are about fifteen in number, become 
pupae at the beginning of June, and the imagos appear towards the 
end of the same month. These are all small ' workers,' and they 
perform such good service in tending the second brood, that the 
latter attain the size of the female which founded the colony ; only 
differing from her in the perfect condition of their wings, for by 
this time her wings are greatly worn away. 

The males appear at the beginning of July ; their spermatozoa 
are mature in August, and pairing then takes place with certain 
' special females which require fertilization ' which have in the 
meantime emerged from their cocoons. These are the females which 
live through the winter and found new colonies in the following 
spring. The old females of the previous winter die, and do not live 

^ E. Bevan, ' Ueber die Honigbiene und die Lange ihres Lebens ; ' abstract in 
Oken's 'Isis,' 1844, p. 506. 



54 THE DURATION OF LIFE. 

beyond the summer at the beginning of which they founded 
colonies. At the first appearance of frost, the young fertilized 
females seek out winter quarters ; the males which never survive 
the winter, do not take this course, but perish in October. The 
parthenogenetic females, which remain in the nest during the 
nuptial flight, also perish. 

The males of Polistes gallica do not live longer than three 
months — from July to the beginning of October ; the partheno- 
genetic females live a fortnight longer at the outside — from the 
middle of June to October, but the later generations have a shorter 
life. The sexual females alone live for about a year, including the 
winter sleep. 

A similar course of events takes place in the genus Vegpa. In 
both these genera the possibility of reproduction is not restricted to 
a single female in the nest, but is shared by a number of females. 
In the genus Apis alone is the division of labour complete, so that 
only a single female (the queen) is at any one time capable of re- 
production, a power which differentiates it from the sterile workers. 

Note 4. The Duration of Life of the Lower Marine Animals, 

I have only met with one definite statement in the literature of 
this part of the subject. It concerns a sea anemone, — which is a 
solitary and not a colonial form. The English zoologist Dalyell, in 
August, 1838, removed an Actinia mesemhryanthemum from the sea 
and placed it in an aquarium ^. It was a very fine individual, 
although it had not quite attained the largest size ; and it must 
have been at least seven years old, as proved by comparison with 
other individuals reared from the &^^. In the year 1848, it was 
about thirty years old, and in the twenty years during which it had 
been in captivity it had produced 334 young Actiniae. Prof. 
Dohrn, of Naples, tells me that this Actinia is still Kving to-day, 
and is shown as a curiosity to those who visit the Botanical Gardens 
in Edinburgh. It is now (1882) at least sixty-one years old 2. 

* Dalyell, ' Rare and Remarkable Animale of Scotland,' vol. ii. p. 203 ; London, 
1848. 

[" Mr. J. S. Haldane has kindly obtained details of the death of the sea anemone 
referred to by the author. It died, by a natural death, on August 4, 1887, after 
having appeared to become gradually weaker for some months previous to this date. 
It had lived ever since 1828 in the same small glass jar in which it was placed by 
Sir John Dalyell. It must have been at least 66 years old when it died. — E.B.P.] 



appendix. 55 

Note 5. The Duration of Life in Indigenous Terrestrial 
AND Fresh-water Mollusca. 

I am indebted to Herr Clessin — the celebrated student of our 
mollusca — for some valuable notes upon our indigenous snails and 
bivalves [Lamellibranchiata). I could not incorporate them in the 
text, for a number of necessary details as to the conditions of life 
are at present entirely unknown, or are at least only known in a 
very fragmentary manner. No statistics as to the amount of de- 
struction suffered by the young are available, and even the number 
of eggs produced annually is only known for a few species. I 
nevertheless include Herr Clessin's very interesting communica- 
tions, as a commencement to the life statistics of the Mollusca. 

(i) ' Vitrinae are annual ; the old animals die in the spring, after 
having produced the spawn from which the young develope. These 
continue to grow until the following spring.' 

(3) 'The Succineae are mostly biennial ; Siiccinea putris probably 
triennial. Fertilization takes place from June till the beginning of 
August, and the young develope until the autumn. Siiccinea Pfeif- 
feri and S. elegans live through the winter, and the fact is proved 
by very distinct annual markings. Reproduction takes place in 
July and August of the following year, and they die in the autumn. 
They continue to grow until their death.' 

(3) ' The shells of our native species of Pupa, Clausilia, and Buli- 
mus (with the exception of Bulimus detritus) show but faint annual 
markings. They can hardly require more than two years for their 
complete development. The great number of living individuals 
with full-sized shells belonging to these genera, as compared with 
the number which possess smaller shells, makes it probable that 
these animals live in the mature condition longer than our other 
Helicidae. I have always found full-sized shells present in at 
least two-thirds of the individuals of these genera characterized by 
much-coiled shells — a proportion which I have never seen among 
our larger Helicidae. Nevertheless direct observations as to the 
length of life in the mature condition are still wanting.' 

(4) ' The Helicidae live from two to four years ; Helix sericea, H. 
hispida, two to three years ; H. hortensis, H. nemoralis, H. arlustonim, 
as a rule three years ; H. pomatia four years. Fertilization is not 
in these species strictly confined to any one time of year, but in the 



56 THE DURATION OF LIFE. 

case of old animals takes place in the spring, as soon as the winter 
sleep is over; while in the two-year-old animals it also happens 
later in the summer.' 

(5) ' The Hyalineae are mostly biennial : they seldom live three 
years, and even in the largest species such an age is probably 
exceptional. The smallest Hyalineae and Helicidae live at most two 
years. The length of life is dependent upon the time at which the 
parents are fertilized, for this decides whether the young begin to 
shift for themselves early in the summer or later in the autumn, 
and so whether the first year's growth is large or small.' 

(6) ' The species of Linmaeus, Planorbis, and Ancylus live two 
to three years, that is they take two to three years to attain the full 
size. L. auricularis is mostly biennial, L. pahistris and L. pereger 
two to three years : I have found that the latter, in the mountains 
at Oberstorf in the Bavarian Alps, may exceptionally attain the 
age of four years, that is, it may possess three clearly defined annual 
markings, whilst the specimens from the plain never showed more 
than two.' 

(7) ' The Paludinidae attain an age of three or four years.' 

(8) ' The smaller bivalves, Pisidium and Cyclas, do not often live 
for more than two years : the larger Najadae, on the other hand, 
often live for more than ten years, and indeed they are not full 
grown until they possess ten to fourteen annual markings. It is 
possible that habitat may have great influence upon the length of 
life in this order.' 

' Unio and Anodonta become sexually mature in the third to the 
fifth year.' 

As far as I am aware but few statements exist upon the length 
of life in marine mollusca, and these are for the most part very 
inexact. The giant bivalve Tridacna gig as must attain an age of 
60 to 100 years ^. All Cephalopods live for at least over a year, 
and most of them well over ten years ; and the giant forms, 
sometimes mistaken for * sea-serpents,' must require man}"- decades 
in which to attain such a remarkable size. L. Agassiz has deter- 
mined the length of life in a large sea snail, Natica heros^ by 
sorting a great number of individuals according to their sizes : he 
places it at 30 years ^. 

' Bronn, ' Klassen und Ordnungen des Thierreichs,' Bd. III. p. 466 ; Leipzig. 
' Bronn, 1. c. 



APPENDIX. 57 

I am g-lad to be able to communicate an observation made at 
the Zoological Station at Naples upon the length of life in 
Ascidians. The beautiful white Cionea intestinalis has settled in 
great numbers in an aquarium at the Station, and Professor Dohrn 
tells me that it produces three generations annually, and that 
each individual lives for about five months, and then reproduces 
itself and dies. External conditions accounting for this early- 
death have not been discovered. 

It is known that the freshwater Tolyzoa are annual, but it is 
not known whether the first individuals produced from a colony 
in the spring, live for the whole summer. The length of life 
is also unknown in single individuals of any marine Polyzoon. 

Clessin's accurate statements upon the freshwater Mollusca, pre- 
viously quoted, show that a surprisingly short length of life is the 
general rule. Only those forms of which the large size requires that 
many years shall elapse before the attainment of sexual maturity, 
live ten years or over {Unio, Anodonta) ; indeed, our largest 
native snail [Helix pomatia) only lives for four years, and many 
small species only one year, or two years if the former time is in- 
sufficient to render them sexually mature. These facts seem to 
indicate, as I think, that these molluscs are exposed to great de- 
struction in the adult state, indeed to a greater extent than when 
they are young, or, at any rate, to an equal extent. The facts 
appear to be the reverse of those found among birds. The 
fertility is enormous ; a single mussel contains several hundred 
thousand eggs ; the destruction of young as compared with the 
number of eggs produced is distinctly smaller than in birds, there- 
fore a much shorter duration of the life of each mature individual 
is rendered possible, and further becomes advantageous because the 
mature individuals are exposed to severe destruction. 

However it can only be vaguely suggested that this is the case, 
for positive proofs are entirely absent. Perhaps the destruction of 
single mature individuals does not play so important a part as the 
destruction of their generative organs. The ravages of parasitic 
animals {Trematodes) in the internal organs of snails and bivalves 
are well known to zoologists. The ovaries of the latter are often 
entirely filled with parasites, and such animals are then incapable 
of reproduction. 

Besides, molluscs have many enemies, which destroy them both 



58 THE DURATION OF LIFE. 

on land and in water. In tbe water, — fish, frogs, newts, ducks and 
other water-fowl, and on land many birds, the hedgehog, toads, etc., 
largely depend upon them for food. 

If the principles developed in this essay apply to the freshwater 
Mollusca, we must then infer that snails which maintain the 
mature condition — the capability of reproduction — for one year, 
are in this state more exposed to destruction from the attacks of 
enemies than those species which remain sexually mature for two 
or three years, or that the latter suffer from a greater proportional 
loss of eggs and young. 

Note 6. Unequal Length of Life in the two Sexes. 

This inequality is frequently found among insects. The males 
of the remarkable little parasites infesting bees, the Strepsiptera, 
only live for two to three hours in the mature condition, while 
the wingless, maggot-like, female Kves eight days : in this case, 
therefore, the female lives sixty-four times as long as the male. 
The explanation of these relations is obvious ; a long life for the 
male would be useless to the species, while the relatively long life 
of the female is a necessity for the species, inasmuch as she is 
viviparous, and must nourish her young until their birth. 

Again, the male of Phylloxera vastatrix lives for a much shorter 
period than the female, and is devoid of proboscis and stomach, and 
takes no food : it fertilizes the female as soon as the last skin has 
been shed and then dies. 

Insects are not the only animals among which we find inequality 
in the length of life of the two sexes. Very little attention has 
been hitherto directed to this matter, and we therefore possess 
little or no accurate information as to the duration of life in 
the sexes, but in some cases we can draw inferences either from 
anatomical structure or from the mode of development. Thus, 
male Rotifers never possess mouth, stomach, or intestine, they 
cannot take food, and without doubt live much shorter lives 
than the females, which are provided with a complete alimentary 
canal. Again, the dwarf males of many parasitic Copepods — • 
low Crustacea — and the ' complementary males ' of Cirrhipedes 
(or barnacles) are devoid of stomach, and must live for a much 
shorter time than the females ; and the male Etttoniscidae (a family 



APPENDIX. 59 

of which the species are endo-parasitic in the larger Crustacea), 
although they can feed, die after fertilizing the females ; while 
the latter then take to a parasitic life, produce eggs, and continue 
to live for some time. It is supposed that the dwarf male of 
Bonellia viridis does not live so long by several years as the hun- 
dred times larger female, and it too has no mouth to its alimentary 
canal. These examples might be further increased by reference to 
zoological literature. 

In most cases the female lives longer than the male, and this 
needs no special explanation : but the converse relation is con- 
ceivable, when, for instance, the females are much rarer than the 
males, and the latter lose much time in seeking them. The above- 
mentioned case of Aglia tau probably belongs to this category. 

We cannot always decide conclusively whether the life of one 
sex has been lengthened or that of the other shortened ; both 
these changes must have taken place in different cases. There is 
no doubt that a lengthening of life in the female has arisen in 
the bees and ants, for both sexes of the saw-flies, which are be- 
lieved to be the ancestors of bees, only live for a few weeks. But 
among the Strej)siptera the shorter life of the male must have been 
secondarily acquired, since we only rarely meet with such an ex- 
treme case in insects. 

Note 7. Bees. 

It has not been experimentally determined whether the workers, 
which are usually killed after some months, would live as long as 
the queen, if they were artificially protected from danger in the 
hive ; but I think that this is probable, because it is the case 
among ants, and because the peculiarity of longevity must be 
latent in the egg. As is well known, the egg which gives rise to 
the queen is identical with that which produces a worker, and 
differences in the nutrition alone decide whether a queen or a 
worker shall be formed. It is therefore probable that the duration 
of life in queen and worker is potentially the same. 

Note 8. Death oe the Cells in higher Organisms. 

The opinion has been often expressed that the inevitable appear- 
ance of normal ' death ' is dependent on the wearing out of the 



60 THE DURATION OF LIFE. 

tissues in consequence of their functional activity. Bertin says, 
referring- to animal life* : — ' L'observation des faits y attache I'idee 
d'une terminaison fatale, bien que la raison ne decouvre nullement 
les motifs de cette necessite. Chez les etres qui font partie du 
regne animal I'exercise meme de la renovation moleculaii-e Unit 
par user le principe qui I'entretient sans doute parceque le <;ra- 
vail d'echang-e ne s'accomplissant pas avec une perfection mathe- 
matique, il s'etabHt dans la figure, comme dans la substance de 
I'etre vivant, une deviation insensible, et que I'accumulation des 
ecarts finit par amener un type chimique ou morphologique in- 
compatible avec la pei'sistance de ce travail.' 

Here the replacement of the used-up elements of tissue by new- 
ones is not taken into account, but an attempt is made to show 
that the functions of the whole organism necessarily cause it to 
waste away. But the question at once arises, whether such a result 
does not depend upon the fact that the sing'le histological elements, 
— the cells, — are worn out by the exercise of function. Bertin 
admits this to be the case, and this idea of the importance of 
changes in the cells themselves is everywhere gaining ground. 
But although we must admit that the histological elements do, as 
a matter of fact, wear out, in multicellular animals, this would not 
prove that, nor explain why, such changes must follow from the 
nature of the cell and the vital processes which take place within 
it. Such an admission would merely suggest the question : — how 
is it that the cells in the tissues of higher animals are worn out 
by their function, while cells which exist in the form of free and 
independent organisms possess the power of living for ever ? Why 
should not the cells of any tissue, of which the equilibrium is 
momentarily disturbed by metabolism, be again restored, so that the 
same cells continue to perform their functions for ever : — why cannot 
they live without their properties suffering alteration ? I have not 
sufficiently touched upon this point in the text, and as it is obviously 
important it demands further consideration. 

In the first place, I think we may conclude with certainty from the 
unending duration of unicellular organisms, that such wearing out 
of tissue cells is a secondary adaptation, that the death of the cell, 
like general death, has arisen with the complex, higher organisms. 
Waste does not depend upon the intrinsic nature of the cells, as the 
* Cf. the article * Mort' in the • Encyclop. Scienc MM.' vol. M. p. 520. 



APPENDTX, 61 

primitive organisms prove to us, but it has appeared as an adapta- 
tion of the cells to the new conditions by which they are surrounded 
when they come into combination, and thus form the cell-republic 
of the metazoan body. The replacement of cells in the tissues must 
be more advantageous for the functions of the whole organism than 
the unlimited activity of the same cells, inasmuch as the power 
of single cells would be much increased by this means. In certain 
cases, these advantages are obvious, as for example in many glands 
of which the secretions are made up of cast-off cells. Such cells 
must die and be separated from the organism, or the secretion would 
come to an end. In many cases, however, the facts are obscure, 
and await physiological investigation. But in the meantime we 
may draw some conclusions from the effects of growth, which are 
necessarily bound up with a certain rate of production of new cells. 
In the process of growth a certain degree of choice between the old 
cells which have performed their functions up to any particular 
time, and the new ones which have appeared between them, is as it 
were left to the organism. 

The organism may thus, figuratively speaking, venture to demand 
from the various specific cells of tissues a greater amount of work 
than they are able to bear, during the normal length of their life, 
and with the normal amount of their strength. The advantages 
gained by the whole organism might more than compensate for 
the disadvantages which follow from the disappearance of single 
cells. The glandular secretions which are composed of cell-de- 
tritus, prove that the cells of a complex organism may acquire 
functions which result in the loosening of their connexion with the 
living cell-community of the body, and their final separation from 
it. And the same facts hold with the blood corpuscles, for the 
exercise of their function results in ultimate dissolution. Hence it 
is not only conceivable, but in every way probable, that many 
other functions in the higher organisms involve the death of the 
cells which perform them, not because the living cell is necessarily 
worn out and finally killed by the exercise of any ordinary vital 
process, but because the specific functions in the economy of the 
cell community which such cells undertake to perform, involve the 
death of the cells themselves. But the fact that such functions 
have appeared, — involving as they do the sacrifice of a great num- 
ber of cells, — entirely depends upon the replacement of the old 



62 THE DURATION OF LIFE. 

by newly formed cells, that is by the process of reproduction in 
cells ^. 

We cannot a priori dispute the possibility of the existence of 
tissues in which the cells are not worn out by the performance of 
function, but such an occurrence appears to be improbable when 
we recollect that the cells of all tissues owe their constitution 
to a very far-reaching process of division of labour, which leaves 
them comparatively one-sided, and involves the loss of many pro- 
perties of the unicellular, self-sufficient organism. At any rate we 
only know of potential immortality in the cells which constitute 
independent unicellular organisms, and the nature of these is such 
that they are continually undergoing a complete process of re- 
formation. 

If we did not find any replacement of cells in the higher 
organism, we should be induced to look upon death itself as the 
direct result of the division of labour among the cells, and to con- 
clude that the specific cells of tissues have lost, as a consequence 
of the one-sided development of their activities, the power of un- 
ending life, which belongs to all independent primitive cells. We 
should argue that they could only perform their functions for a 
certain time, and would then die, and with them the organism whose 
life is dependent upon their activity. The longer they are occupied 
with the performance of special functions, the less completely do 
they carry out the phenomena of life, and hence they lead to the 
appearance of retrogressive changes. But the replacement of cells 
is certain in many tissues (in glands, blood, etc), so that we can 
never seek a satisfactory explanation in the train of reasoning in- 
dicated above, but we must assume the existence of limits to the 
replacement of cells. In my opinion, we can find an explanation 
of this in the general relations of the single individual to its 
species, and to the whole of the external conditions of life ; and this 
is the explanation which I have suggested and have attempted to 
work out in the text. 

1 Boux, in his work ' Der Kampf der Theile im Organismus,' Jena 1881, has 
attempted to explain the manner in which division of labour has arisen among the 
cells of the higher organisms, and to render intelligible the mechanical processes 
by which the purposeful adaptations of the organism have arisen. 



appendix. 63 

Note 9. Death by Sudden Shock. 

The most remarkable example of this kind of death known to me, 
is that of the male bees. It has been long known that the drone 
perishes while pairing, and it was usually believed that the queen 
bites it to death. Later observations have however shown that 
this is not the case, but that the male suddenly dies during copulation, 
and that the queen afterwards bites through the male intromittent 
organ, in order to free herself from the dead body. In this case 
death is obviously due to sudden excitement, for when the latter is 
artificially induced, death immediately follows. Von Berlepsch 
made some very interesting observations on this point, * If one 
catches a drone by the wings, during the nuptial flight, and holds 
it free in the air without touching any other part, the penis is pro- 
truded and the animal instantly dies, becoming motionless as 
though killed by a shock. The same thing happens if one gently 
stimulates the dorsal surface of the drone on a similar occasion. 
The male is in such an excited and irritable condition that the 
slightest muscular movement or disturbance causes the penis to 
be protruded^.' In this case death is caused by the so-called 
nervous shock. The humble-bees are not similarly constituted, for 
the male does not die after fertilizing the female, ' but withdraws 
its penis and flies away.' But the death of male bees, during 
pairing, must not be regarded as normal death. Experiment has 
shown that these insects can live for more than four months^. 
They do not, as a matter of fact, generally live so long ; for — 
although the workers do not, as was formerly believed, kill them 
after the fertilization of the queen, by direct means — they prevent 
them from eating the honey and drive them from the hive, so that 
they die of hunger ^. 

We must also look upon death which immediately, or very 
quickly, follows upon the deposition of eggs as death by sudden 
shock. The females of certain species of Psi/cJddae, when they re- 
produce sexually, may remain alive for more than a week waiting 
for a male : after fertilization, however, they lay their eggs and 
die, while the parthenogenetic females of the same species lay their 

1 von Berlepsch, ' Die Biene und ihre Zucht,' etc. 
'^ Oken, ' Isis,' 1844, p. 506. 
^ von Berlepsch, 1. c, p. 165. 



€4 THE DURATION OF LIFE. 

eggs and die immediately after leaving the cocoon ; so that while 
the former live for many days, the latter do not last for more than 
twenty-four hours. ' The parthenogenetic form of Solenobia tri- 
qneirella, soon after emergence, lays all her eggs together in the 
empty case, becomes much shrunken, and dies in a few hours.' 
(Letter from Dr. Speyer, Rhoden.) 



Note io. Intermingling during the Fission op Unicellulae 

Organisms ^ 

Fission is quite symmetrical in Amoebae, so that it is impossible 
to recognise mother and daughter in the two resulting organisms. 
But in Etiglypha and allied forms the existence of a shell 
introduces a distinguishing mark by which it is possible to 
discriminate between the products of fission ; so that the offspring 
can be differentiated from the parent. The parent organism, 
before division, builds the parts of the shell for the daughter form. 
These parts are arranged on the surface of that part of the proto- 
plasm, external to the old shell, which will be subsequently separated 
as the daughter-cell. On this part the spicules are ai-ranged and 
unite to form the new shell. The division of the nucleus takes 
place after that of the protoplasm, so that the daughter-cell is for 
some time without a nucleus. Although we can in this species 
recognise the daughter-cell for some time after separation from 
the parent by the greater transparency of its younger shell, it is 
nevertheless impossible to admit that the characteristics of the two 
animals are in any way different, for just before the separation of 
the two individuals a circulation of the protoplasm through both 
shells takes place after the manner described in the text, and 
there is therefore a complete intermingling of the substance of 
the two bodies. 

The difference between the products is even greater after trans- 
verse fission of the Infusoria, for a new anus must be formed at 
the anterior part and a new mouth posteriorly. It is not known 
whether any circulation of the protoplasm takes place, as in Eii- 
glypha. But even if this does not occur, there is no reason for 

* Cf. August Gruber, 'Der Theilungsvorgang bei Euglypha alveolata,' and 'Die 
Theilung der monothalamen Rhizopoden,' Z. f. W. Z., Bd. XXXV. and XXXVI., 
p. 104, 1881. 



APPENDIX. 65 

believing' that the two products of division possess a different dura- 
tion of life. 

The process of fission in the Diatomaceae seems to me to be 
theoretically important, because here, as in the previously-mentioned 
Monothalamia [Eiigli/pJia, etc.), the new silicious skeleton is built 
up within the primary organism, but not, as in Eiiglijpha, for the 
new individual only, but for both parent and daughter-cell alike ^. 
If we compare the diatom shell to a box, then the two halves of 
the old shell would form two lids, one for each of the products of 
fission, while a new box is built up afresh for each of them. In 
this case there is an absolute equality between the products of 
fission, so far as the shell is concerned. 

Note ii. Regenekation. 

A number of experiments have been recently undertaken, in 
connection with a prize thesis at Wiirzburg, in order to test the 
powers of regeneration possessed by various animals. In all essen- 
tial respects the results confirm the statements of the older 
observers, such as Spallanzani. Carriere has also proved that 
snails can regenerate not only their horns and eyes, but also part 
of the head when it has been cut off", although he has shown that 
Spallanzani's old statement that they can regenerate the whole 
head, including the nervous system, is erroneous ^. 

Note 12. The Duration of Life in Plants. 

The title of the work on this subject mentioned in the Text is 

* Die Lebensdauer und Vegetationsweise der Pflanzen, ihre Ursache 
und ihre Entwicklung,' F. Hildebrand, Engler's botanische Jahr- 
biicher, Bd. II. i. und a. Heft, Leipzig, 1881. 

Note 13. 

[Many interesting facts and conclusions upon the subject of this 
essay will be found in a volume by Professor E. Ray Lankester, 

* On comparative Longevity in Man and the lower Animals,' Mac- 
millan and Co., 1870. — E. B. P.] 

^ Cf. Victor Hensen, ' Physiologie d. Zeugung,' p. 152. 

^ Cf. J. Carrifere, 'TJeber Kegeneration bei Landpulmonaten,' Tagebl. der 52. 
Versammlg. deutscb. Naturf. pp. 225-226. 

F 



n. 

ON HEEEDITY. 

1883. 



P 2 



ON HEREDITY. 



PREFACE. 

The following essay was my inaugural lecture as Pro-Rector of 
the University of Freiburg, and was delivered publicly in the hall 
of the University, on June 2i, 1883 ; it first appeared in print in 
the following August. Only a few copies of the first edition were 
available for the public, and it is therefore now reprinted as a second 
edition, which only differs from the first in a few not unimportant 
improvements and additions. 

The title which I have cbosen requires some explanation, I do 
not propose to treat of the whole problem of heredity, but only 
of a certain aspect of it — the transmission of acquired characters 
which has been hitherto assumed to occur. In taking this course 
I may say that it was impossible to avoid going back to the 
foundation of all the phenomena of heredity, and to determine the 
substance with which they must be connected. In my opinion 
this can only be the substance of the germ-cells ; and this sub-. 
stance transfers its hereditary tendencies from generation tflL^gfli. 
neration^at first unchajftged, and always uninfluenced in any corre- 
sponding manner, by that which happens during the life of the 
individual which bears it. If these views, which are indicated 
rather than elaborated in this paper, be correct, all our ideas upon 
the transformation of species require thorough modification, for the 
whole principle of evolution by means of exercise (use and disuse), 
as proposed by Lamarck, and accepted in some cases by Darwin, 
entirely collapses. 

The nature of the present paper — which is a lecture and not 
an elaborate treatise — necessitates that only suggestions and not 



70 ON HEREDITY. 

an exhaustive treatment of the subject could be given. I have also 
abstained from giving further details in the form of an appendix, 
chiefly because I could hardly have attempted to complete a treat- 
ment of the whole range of the subject, and I hope to refer 
again to these questions in the future, when new experiments and 
observations have been made. 

I am very glad to see that such an important authority as 
Pfliiger^ has in the meantime come to the same opinion, from an 
entirely different direction — an opinion which forms the founda- 
tion of the views here brought forward, namely, that heredity 
depends upon the continuity of the molecular substance of the 
germ from generation to generation. 

A. W. 

* Pfluger, * Ueber den Einfluss der Schwerkraft auf die Theilung der Zellen und 
auf die Entwicklung des Embryo/ Arch. f. Physiol. Bd. XXXII. p, 68, 1883. 



11. 

ON HEREDITY. 

"With your permission I wish to bring before you to-day my 
views on a problem of general biological interest — the problem of 
heredity. 

Heredity is the process which renders possible that persistence 
of organic beings throughout successive generations, which is 
generally thought to be so well understood and to need no special 
explanation. Nevertheless our minds cannot fail to be much per- 
plexed by the multiplicity of its manifestations, and to be greatly 
puzzled as to its real nature. A celebrated German physiologist 
says ^5 'Although many hands have at all times endeavoured to 
break the seal which hides the theory of heredity from our view, 
the results achieved have been but small ; and we are in a certain 
degree justified in looking with little hope upon new efforts under- 
taken in this direction. We must nevertheless endeavour from 
time to time to ascertain how far we have advanced towards a 
complete explanation.' 

Such a course is in every way advisable, for we are not dealing 
with phenomena which frpm their very nature are incomprehensible 
by man. The great complexity of the subject has alone rendered it 
hitherto insuperable, but in the province of heredity we certainly 
have not reached the limits of attainable knowledge. 

From this point of view heredity bears some resemblance to cer- 
tain anatomical and physiological problems, e. g. the structure and 
function of the human brain. Its structure — with so many millions 
of nerve- fibres and nerve-cells — is of such extraordinary complexity 
that we might well despair of ever completely understanding it. 
Each fibre is nevertheless distinct in itself, while its connection 
with the nearest nerve-cell can be frequently traced, and the function 
of many groups of cell elements is already known. But it would 
seem to be impossible to unravel the excessively complex network 
^ Victor Hensen in his ' Physiologie der Zeugung,' Leipzig, 1881, p. 216. 



72 ON HEREDITY. 

into which the cells and fibres are knit together ; and hence to 
arrive at the function of each sing-le element appears to be also 
beyond our reach. We have not however commenced to untie 
this Gordian knot without some hope of success, for who can say 
how far human perseverance may be able to penetrate into the 
mechanism of the brain, and to reveal a connected structure and 
a common principle in its countless elements? But surely this 
work will be most materially assisted by the simultaneous in- 
vestigation of the structure and function of the nervous system in 
the lower forms of life — in the polypes and jelly-fish, worms and 
Crustacea. In the same way we should not abandon the hope of 
arriving at a satisfactory knowledge of the processes of heredity, 
if we consider the simplest processes of the lower animals as well 
as the more complex processes met with in the higher forms. 

The word heredity in its common acceptation, means that pro- 
perty of an organism by which its peculiar nature is transmitted to . 
its descendants. From an eagle's egg an eagle of the same species 
developes ; and not only are the characteristics of the species 
transmitted to the following generation, but even the individual 
peculiarities. The offspring resemble their pai'ents among animals 
as well as among men. 

On what does this common property of all organisms depend ? 

Hackel w^as probably the first to describe reproduction as 'an over- 
growth of the individual,' and he attempted to explain heredity as a 
simple continuity of growth. This definition might be considered 
as a play upon words, but it is more than this ; and such an inter- 
pretation rightly applied, points to the only path which, in my 
opinion, can lead to the comprehension of heredity. 

Unicellular organisms, such as Rhizopoda and Infusoria, increase 
by means of fission. Each indi\ddual grows to a certain size, and 
then divides into two parts, which are exactly alike in size and 
structure, so that it is impossible to decide whether one of them 
is younger or older than the other. Hence in a certain sense these 
organisms possess immortality : they can, it is true, be destroyed, 
but, if protected from a violent death, they would live on in- 
definitely, and would only from time to time reduce the size of 
their overgrown bodies by division. Each individual of any such 
unicellular species living on the earth to-day is far older than man- 
kind, and is almost as old as life itself. 



ON HEREDITY. 73 

From these unicellular org-anisms we can to a certain extent 
understand why the offspring-, being- in fact a part of its parents, 
must therefore resemble the latter. The question as to why the 
part should resemble the whole leads us to a new problem, that of 
assimilation, which also awaits solution. It is, at any rate, an 
undoubted fact that the organism possesses the power of taking- up 
certain foreign substances, viz. food, and of converting them into 
the substance of its own body. 

Among these unicellular organisms, heredity depends upon the 
continuity of the individual during the continual increase of its 
body by means of assimilation. 

But how is it with the multicellular organisms which do not 
reproduce by means of simple division, and in which the whole 
body of the parent does not pass over into the oifspring ? 

In such animals sexual reproduction is the chief means of mul- "" 
tiplication. In no case has it always been completely wanting, 
and in the majority of cases it is the only kind of reproduction. 

In these animals the power of reproduction is connected with 
certain cells which, as germ-cells, may be contrasted with those 
which form the rest of the body; for the former have a totally 
different role to play; they are without significance for the life of 
the individual ^, and yet they alone possess the power of preserving 
the species. Each of them can, under certain conditions, develope 
into a complete organism of the same species as the parent, with 
every individual peculiarity of the latter reproduced more or less 
completely. How can such hereditary transmission of the characters 
of the parent take place ? how can a single reproductive cell repro- 
duce the whole body in all its details ? 

Such a question could be easily answered if we were only con- \ 
cerned with the continuity of the substance of the reproductive cells 
from one generation to another ; for this can be demonstrated 
in some cases, and is very probable in all. In certain insects 
the development of the egg into the embryo, that is the segmen- 
tation of the egg, begins with the separation of a few small cells 
from the main body of the egg. These are the reproductive cells, 
and at a later period they are taken into the interior of the 
animal and form its reproductive organs. Again, in certain 
small freshwater Crustacea (Daphndae) the future reproductive 
^ That is for the preservation of its life. 



74 ON HEBEDITY. 

cells become distinct at a very early period, although not quite 
at the beginning of segmentation, i. e. when the egg has divided 
into not more than thirty segments. Here also the cells which are 
separated early form the reproductive organs of the animal. The 
separation of the reproductive cells from those of the body takes 
place at a still later period, viz. at the close of segmentation, in 
Sagitta — a pelagic free-swimming form. In Vertebrata they do 
not become distinct from the other cells of the body until the 
embryo is completely formed. Thus, as their development shows, 
a marked antithesis exists between the substance of the undying 
reproductive cells and that of the perishable body-cells. We 
cannot explain this fact except by the supposition that each re- 
productive cell potentially contains two kinds of substance, which 
at a variable time after the commencement of embryonic develop- 
ment, separate from one another, and finally produce two sharply 
contrasted groups of cells. 

It is evidently unimportant, as regards the question of heredity, 
whether this separation takes place early or late, inasmuch as the 
molecular constitution of the reproductive substance is determined 
before the beginning of development. In order to understand the 
growth and multiplication of cells, it must be conceded that all 
protoplasmic molecules possess the power of growing, that is of 
assimilating food, and of increasing by means of division. In the 
same manner the molecules of the reproductive protoplasm, when 
well nourished, grow and increase without altering their peculiar 
nature, and without modifying the hereditary tendencies derived 
from the parents. It is therefore quite conceivable that the re-i 
productive cells might separate from the somatic cells much laterj 
than in the examples mentioned above, without changing the! 
hereditary tendencies of which they are the bearers. There may! 
be in fact cases in which such separation does not take place until 
after the animal is completely formed, and others, as I believe that 
I have shown ^, in which it first arises one or more generations 
later, viz. in the buds produced by the parent. Here also there is 
no ground for the belief that the hereditary tendencies of the repro- 
ductive molecules are in any way changed by the length of time 
which elapses before their separation from the somatic molecules. 

' Compare Weismann, * Die Entstehung der Sexualzellen bei den Hydromedusen,' 
Jena, 1883. 



ON HEBEDITY. 75 

And this theoretical deduction is confirmed by observation, for from 
the egg of a Medusa, produced by the budding of a Polype, a 
Polype, in tlie first instance, and not a Medusa arises. Here the 
molecules of the reproductive substance first formed part of the 
Polype, and later, part of the Medusa bud, and, although they 
separated from the somatic cells in the bud, they nevertheless 
always retain the tendency to develope into a Polype. 

We thus find that the reproduction of multicellular organisms is 
essentially similar to the corresponding process in unicellular forms ; 
for it consists in the continual division of the reproductive cell ; 
the only difierence being that in the former case the reproductive 
cell does not form the whole individual, for the latter is composed 
of the millions of somatic cells by which the reproductive cell is 
surrounded. The question, 'How can a single reproductive cell 
contain the germ of a complete and highly complex individual ? ' 
must therefore be re-stated more precisely in the following form, 
' How can the substance of the reproductive cells potentially con- 
tain the somatic substance with all its characteristic properties?' 

The problem which this question suggests, becomes clearer when 
we employ it for the explanation of a definite instance, such as the 
origin of multicellular from unicellular animals. There can be 
no doubt that the former have originated from the latter, and that 
the physiological principle upon which such an origin depended, is 
the principle of division of labour. In the course of the phyletic 
development of the organized world, it must have happened that 
certain unicellular individuals did not separate from one another 
immediately after division, but lived together, at first as equivalent 
elements, each of which retained all the animal functions, including 
that of reproduction. The Magosp/iaera _planula of Hackel proves that 
such perfectly homogeneous cell-colonies exist ^, even at the present 
day. Division of labour would produce a differentiation of the single 
cells in such a colony : thus certain cells would be set apart for ob- 
taining food and for locomotion, while certain other cells would be 
exclusively reproductive. In this way colonies consisting of somatic 
and of reproductive cells must have aiisen, and among these for 

* It is doubtful whether Magosphaera should be looked upon as a mature form ; 
but nothing hinders us from believing that species have lived, and are still living, in 
which the ciliated sphere has held together until the encystment, that is the re- 
production, of the constituent single cells. 



76 ON HEREDITY. 

the first time death appeared. Foi* in each ease the somatic cells 

must have perished after a certain time, while the reproductive 

cells alone retained the immortality inherited from the Protozoa. 

.' We must now ask how it becomes possible that one kind of cell 

; in such a colony can produce the other kind by division ? Before 

i the differentiation of the colony each cell always produced others 

similar to itself. How can the cells, after the nature of one part 

of the colony is changed, have undergone such changes in their 

nature that they can now produce more than one kind of cell ? 

Two theories can be brought forward to solve this problem. We 
may turn to the old and long since abandoned nisus formativug, 
or adapting the name to modern times, to a phyletic force of 
development which causes the organism to change from time to 
time. This vis a tergo or teleological force compels the organism to 
undergo new transformations without any reference to the external 
conditions of life. This theory throws no light upon the numerous 
adaptations which are met with in every organism ; and it possesses 
no value as a scientific explanation. 

Another supposition is that the primary reproductive cells are 
influenced by the secondary cells of the colony, which, by their 
r ,aIj\_ adaptability to the external conditions of life, have become somatic 

y" \ cells : that the latter give off minute particles which entering into 
the former, cause such changes in their nature that at the next 
succeeding cell-division they are compelled to break up into dissimilar 
parts. 

At first sight this hypothesis seems to be quite reasonable. It is 
not only conceivable that particles might proceed from the somatic 
jfco the reproductive cells, but the very nutrition of the latter at the 
xpense of the former is a demonstration that such a passage 
ctually takes place. But a closer examination reveals immense 
difficulties. In the first place, the molecules of the body devoured 
are never simply added to those of the feeding individual without 
undergoing any change, but as far as we know, they are really as- 
similated ^, that is, converted into the molecules of the latter. We 
cannot therefore gain much by assuming that a number of mole- 
cules can pass from the growing somatic cells into the growing 
reproductive cells, and can be deposited unchanged in the latter, so 

• Or is an exception perhaps afforded by the nutritive cells of the egg, which 
occur in many animals ? 



^^ 



ON HEREDITY. It 

that, at their next division, the molecules are separated to become 
the somatic cells of the following generation. How can such a 
process be conceivable, when the colony becomes more complex, 
when the number of somatic cells becomes so large that they 
surround the reproductive cells with many layers, and when at the 
same time by an increasing division of labour a great number of 
different tissues and cells are produced, all of which must originate 
de 710V0 from a single reproductive cell ? Each of these various 
elements must, ex hypothesis give up certain molecules to the re- 
productive cells ; hence those which are in immediate contact with 
the latter would obviously possess an advantage over those which 
are more remote. If then any somatic cell must send the same 
number of molecules to each reproductive celP, we are compelled to 
suspend all known physical and physiological conceptions, and 
must make the entirely gratuitous assumption of an affinity on the 
part of the molecules for the reproductive cells. Even if we admit 
the existence of this affinity, its origin and means of control remain 
perfectly unintelligible if we suppose that it has arisen from 
differentiation of the complete colony. An unknown controlling 
force must be added to this mysterious arrangement, in order to 
marshal the molecules which enter the reproductive cell in such a 
manner that their arrangement corresponds with the order in 
which they must emerge as cells at a later period. In short, we 
become lost in unfounded hypotheses. 

It is well known that Darwin has attempted to explain the 
phenomena of heredity by means of a hypothesis which corresponds 
to a considerable extent with that just described. If we substitute 
gemmules for molecules we have the fundamental idea of Darwin's 
provisional hypothesis of pangenesis. Particles of an excessively 
minute size are continually given off from all the cells of the body; 
these particles collect in the reproductive cells, and hence any 
change arising in the organism, at any time during its life, is repre- 
sented in- the reproductive celj ^. Darwin believed that he had by 
this means rendered the transmission of acquired characters in- 
telligible, a conception which he held to be necessary in order to 

' Or more precisely, they must give up as many molecules as would correspond to 
the number of the kind of cell in question found in the mature organism. 

* See Darwin, 'The Variation of Animals and Plants under Domestication,' 1875, 
vol. ii. chapter xxvii. pp. 349-399- 



78 ON HEREDITY. 

explain the development of species. He himself pointed out that 
the hypothesis was merely provisional, and that it was only an ex- 
pression of immediate, and by no means satisfactory knowledge of 
these phenomena. 

It is always dangerous to invoke some entirely new force in 
order to understand phenomena which cannot be readily explained 
by the forces which are already known. 

I believe that an explanation can in this case be reached by an 

appeal to known forces, if we suppose that characters acquired (in the 

I true sense of the term) by the parent cannot appear in the course 

of the development of the offspring, but that all the characters 

exhibited by the latter are due to primary ohangps in tjip germ. 

This supposition can obviously be made with regard to the 
above-mentioned colony with its constituent elements differentiated 
into somatic and reproductive cells. It is conceivable that the 
differentiation of the somatic cells was not primarily caused by a 
change in their own structure, but that it was prepared for by 
changes in the molecular structure of the reproductive cell from 
which the colony ai'ose. 

The generally received idea assumes that changes in the external 
conditions can, in connection with natural selection, call forth per- 
sistent changes in an organism ; and if this view be accepted it 
must be as true of all Metazoa as it is of unicellular or of homo- 
geneous multicellular organisms. Supposing that the hypothe- 
tical colonies, which were at first entirely made up of similar cells, 
were to gain some advantages, if in the course of deM^^pment, the 
molecules of the reproductive cells, from which each colony arose 
became distributed irregularly in the resulting organism, there 
would be a tendency towards the perpetuation of such % change, 
•i wherever it appeared as the result of individual variability. As a 
j result of this change the colony would no longer remain homo- 
j geneous, and its cells would become dissimilar from the first, 
^ because of the altered arrangement of the molecules in the repro- 
ductive cells. Nothing prevents us from assuming that, at the 
same time, the nature of a part of the molecule may undergo still 
further change, for the molecules are by nature complex, and may 
split up or combine together. 

If then the reproductive cells have undergone such changes that 
they can produce a heterogeneous colony as the result of continual 



ON HEREDITY. 79 

division, it follows that succeediDg* generations must behave in 
exactly the same manner, for each of them is developed from a 
portion of the reproductive cell from which the previous generation 
arose, and consists of the same reproductive substance as the 
latter. 

From this point of view the exact manner in which we imagine 
the subsequent differentiatipn of the colony to be potentially pre- 
sent in the reproductive cgll, becomes a matter of comparatively 
small importance. It may consist in a d ifferent m olec ular arran ge- 
ment, or in jODie_cliaJig-e-Qf-ch£ mical constitutio n, or it may be due 
to both these causes combined. The essential point is that the dif- 
ferentiation was originally due to somie change.in the reproduoMxP 
cells, just as this change itself produces all the differentiations which 
appear in the ontogeny of all species at the present day. No one 
doubts that the reason why this or that form of segmentation takes 
place, or why this or that species finally appears, is to be found in 
the ultimate structure of the reproductive cells. And, as a matter of 
fact, molecular differentiation and grouping, whether present from 
the beginning or first appearing in the course of development, 
plays a role which can be almost directly observed in certain 
species. The first segmentation fm-row divides the egg of such 
species into an opaque and a clear half, or, as is often the case 
among Medusae, into a granular outer layer and a clear central 
part, corresponding respectively with the ectoderm and endoderm 
which are formed at a later period. Such early differentiations are 
only the visible proofs of certain highly complex molecular re- 
arrangements in the cells, and the fact appears to indicate that we 
cannot be far wrong in maintaining that differentiations which 
appear in the course of ontogeny depend upon the chemical and 
physical constitution of the molecules in the reproductive cell. 

At the first appearance of the earliest Metazoa alluded to above, 
only two kinds of cells, somatic and reproductive, arose from the 
segmentation of the reproductive cell. The reproductive cells thus 
formed must have possessed exactly the same molecular structure 
as the mother reproductive cell, and would therefore pass through 
precisely the same developmental changes. We can easily imagine 
that all the succeeding stages in the development of the Metazoa 
have been due to the same causes which were efficient at the 
earliest period. Variations in the molecular structure -of the 



80 ON HEREDITY. 

reproductive cells jaraiild continue to appear,- andHshese-would-be 
iQcreased^aniL rendered, permaneGL b^ means of natural selp-ctionT 
when their results, in the alteration of certain cells in the body, 
I were advantageous to the species. The only condition necessary 
for the transmission of such changes is that a part _e£_the_repro- 
ductive sub stance (the germ-plasm) should always rgnnftiTi nnnhange d 
during"segmentation and the subsequent building up of the body, 
or in other words, that such unchanged substance should pass into 
the organism, and after the lapse of a variable period, should reappear 
as the reproductive cells. Only in this way can we render to some 
extent intelligible the transmission of those changes which have 
arisen in the phylogeny of the species ; only thus can we imagine 
the manner in which the first somatic cells gradually developed 
in numbers and in complexity. 

It is only by supposing that these changes arose from molecular 
alterations in the reproductive cell that we can understand how the 
reproductive cells of the next generation can originate the same 
changes in the cells which are developed from them ; and it is 
^impossible to imagine any way in which the transmission of changes, 
produced by the direct action of external forces upon the somatic 
cells, can be brought about ^. 

The difficulty or the impossibility of rendering the transmission 
of acquired characters intelligible by an appeal to any known force 
has been often felt, but no one has hitherto attempted to cast doubts 
upon the very existence of such a form of heredity. 

There are two reasons for this : first, observations have been 
recorded which appear to prove the existence of such transmission ; 
and secondly, it has seemed impossible to do without the supposition 
of the transmission of acquired characters, because it has always 
played such an important part in the explanation of the trans- 
formation of species. 

It is perfectly right to defer an explanation, and to hesitate 

* To this class of phenomena of course belong those acts of will which call forth 
the functional activity of certain groups of cells. It is quite clear that such im- 
pulses do not originate in the constitution of the tissue in question, but are due to the 
operation of external causes. The activity does not arise directly from any natural 
disposition of the germ, but is the result of accidental external impressions. A 
domesticated duck uses its legs in a different manner from, and more fret^uently than 
a wild duck, but such functional changes are the consequence of changed external 
conditions, and are not due to the constitution of the germ. 



ON HEKEDITY. 81 

before we declare a supposed phenomenon to be impossible, because 
we are unable to refer it to any of the known forces. No one can 
believe that we are acquainted with all the forces of nature. But, 
on the other hand, we must use the greatest caution in dealing 
with unknown forces ; and clear and indubitable facts must be 
brought forward to prove that the supposed phenomena have a real 
existence, and that their acceptance is unavoidable. 

It has never been proved that acquired characters are trans- 
mitted, and it has never been demonstrated that, without the aid 
of such transmission, the evolution of the organic world becomes 
unintelligible. 

The inheritance of acquired characters has never been proved, 
either by means of direct observation or by experiment^. It must 
be admitted that there are in existence numerous descriptions of 
cases which tend to prove that such mutilations as the loss of 
fingers, the scars of wounds, etc., are inherited by the offspring, 
but in these descriptions the previous history is invariably obscure, 
cand hence the evidence loses all scientific value^ 

As a typical example of the scientific value of such cases I may 
mention the frequently quoted instance of the cow, which lost its 
left hom from suppuration, induced by some ' unknown cause,' and 
which afterwards produced two calves with a rudimentary left horn 
in each case. But as Hensen^ has rightly remarked, the loss of 
the cow's horn may have arisen from a congenital malformation, 
which would certainly be transmitted, but which was not an ac- 
quired character. 

The only cases worthy of scientific discussion are the well-known 
experiments upon guinea-pigs, conducted by the French physiologist 
Brown- S6quard. But the explanation of his results is, in my 
opinion, open to discussion. In these eases we have to do with 
the apparent transmission of artificially produced malformations. 
The division of important nerves, or of the spinal cord, or the 

^ Upon this subject Pfluger states — ' I have made myself accurately acquainted 
with all facts which are supposed to prove the inheritance of acquired characters, — 
that is of characters which are not due to the peculiar organization of the ovum and 
spermatozoon from which the individual is formed, but which follow from the in- 
cidence of accidental external influences upon the organism at any time in its life. 
Not one of these facts can be accepted as a proof of the transmission of acquired 
characters.' I. c. p. 68. 

* ' Physiologic der Zeugung.' 

G 



82 ON HEREDITY. 

removal of parts of the brain, produced certain symptoms which 
reappeared in the descendants of the mutilated animals. Epilepsy 
was produced by dividing the great sciatic nerve ; the ear became 
deformed when the sympathetic nerve was severed in the throat ; 
and prolapsus of the eye-ball followed the removal of a cei-tain 
part of the brain — the corpora restiformia. All these effects were 
said to be transmitted to the descendants as far as the fifth or sixth 
generation. 

But we must inquire whether these cases are really due to here- 
dity and not to simple infection. In the case of epilepsy, at 
any rate, it is easy to imagine that the passage of some specific 
organism through the reproductive cells may take pla<5e, as in 
the case of syphilis. We are, however, entirely ignorant of the 
nature of the former disease. This suggested explanation may 
not perhaps apply to the other cases : but we must remember that 
animals which have been subjected to such severe operations upon 
the nervous system have sustained a great shock, and if they 
are capable of breeding, it is only probable that they will produce 
weak descendants, and such as are easily affected by disease. Such 
a result does not however explain why the offspring should suffer 
from the same disease as that which was artificially induced in 
the parents. But this does not appear to have been by any means 
invariably the case. Brown-Sequard himself says, ' The changes 
in the eye of the offspring were of a very variable nature, and 
were only occasionally exactly similar to those observed in the 
parents.' 

There is no doubt, however, that these experiments demand 
careful consideration, but before they can claim scientific recogni- 
tion, they must be subjected to rigid criticism as to the precautions 
taken, the number and nature of the control experiments, etc. 

Up to the present time such necessary conditions have not been 
sufficiently observed. The recent experiments themselves are only 
described in short preliminary notices, which, as regards their accu- 
racy, the possibility of mistake, the precautions taken, and the exact 
succession of individuals affected, afford no data upon which a 
scientific opinion can be founded. Until the publication of a com- 
plete series of experiments, we must say with Du Bois Reymond ^, 
* The hereditary transmission of acquired characters remains an 
' See 'Ueber die Uebung,' Berlin, 1881. 



ON HEREDITY. 83 

unintelligible hypothesis, which is onlj deduced from the facts 
which it attempts to explain.' 

We therefore naturally ask whether the hypothesis is really 
necessary for the explanation of known facts. 

At the first sight it certainly seems to be necessary, and it 
appears rash to attempt to dispense with its aid. Many phenomena 
only appear to be intelligible if we assume the hereditary trans- 
mission of such acquired characters as the changes which we 
ascribe to the use or disuse of particular organs, or to the direct 
influence of climate. Furthermore, how can we explain instinct 
as hereditary habit unless it has gradually arisen by the accumula- 
tion, through heredity, of habits which were practised in succeeding 
generations ? 

I will now attempt to prove that even these cases, so far as they 
depend upon clear and indubitable facts, do not force us to accept 
the supposition of the transmission of acquired characters. 

It seems difficult and well nigh impossible to deny the transmis- 
sion of acquired characters when we remember the influence which 
use an d disus e have exercised upon certain special organs. It is 
well known that Lamarck attempted to explain the structure of 
the organism as almost entirely due to this principle alone. Accord- 
ing to his theory the long neck of the giraffe arose by constant 
stretching after the leaves of trees, and the web between the toes 
of a water-bird's foot by the extension of the toes, in an attempt 
to oppose as large a surface of water as possible in swimming. There 
can be no doubt that those muscles which are frequently used 
increase in size and strength, and that glands which often enter 
into activity become larger and not smaller, and that their func- 
tional powers increase. Indeed, the whole efi'ect which exercise 
produces upon the single parts of the body is dependent upon the 
fact that frequently used organs increase in strength. This con- 
clusion also refers to the nervous system, for a pianist who per- 
forms with lightning rapidity certain pre-arranged, highly complex, 
and combined movements of the muscles of his hands and fingers 
has, as Du Bois Reymond pointed out, not only exercised the 
muscles, but also those ganglionic centres of the brain which deter- 
mine the combination of muscular movement. Other functions 
of the brain, such as memory, can be similarly increased and 
strengthened by exercise, and the question to be settled is whether 

G 3 



m>' 



84 ON HEREDITY. 

characters acquired in this way by exercise and practice can be 
transmitted to the following- generations. Lamarck's theory 
assumes that such transmission takes place, for without it no 
accumulation or increase of the characters in question would be 
possible, as a result of their exercise during any number of successive 
generations. 

C Against this we may urge that whenever, in the course of 
nature, an organ becomes stronger by exercise, it must possess a 
certain degree of importance for the life of the individual, and when 
this is the case it becomes subject to improvement by natural 
selection, for only those individuals which possess the organ in its 
most perfect form will be able to reproduce them. The perfection 
of form of an organ does not however depend upon the amount 
of exercise undergone by it during the life of the organism, but 
primarily and principally upon the fact that the genu from which 
the individual arose was predisposed to produce a perfect organ. 
The increase to which any organ can attain by exercise during a 
single life is bounded by cei-tain limits, which are themselves fixed 
by the primary tendencies of the organ in question./ We cannot 
by excessive feeding make a giant out of the germ destined to 
form a dwarf; we cannot, by means of exercise, transform the 
muscles of an individual destined to be feeble into those of a 
Hercules, or the brain of a predestined fool into that of a Leibnitz 
or a Kant, by means of much thinking. With the same amount 
of exercise the organ which is destined to be strong, will attain 
a higher degree of functional activity than one that is destined to 
be weak. Hence natural selection, in destroying the least fitted 
individuals, destroys those which from the germ w(m-o ft^ebly dis- 
posed./' Thus the result of exercise during the individual life does 
not acquire so much importance, for, as compared with differences 
in predisposition, the amount of exercise undergone by all the 
individuals of a species becomes relatively uniform. #" The i ncrease 
ofjiLjorgan in the course of generations does not depend upon 
the summation of the exercise taken during single lives, but 
upon the summation o f more favo urable predispositions in the 
germs. / 

In criticizing these arguments, it may be questioned whether the 
single individuals of a species which is undergoing modification do, 
as a matter of fact, exercise themselves in the same manner and to 



ON HEREDITY. 85 

the same extent. But the consideration of a definite example 
clearly shows that this must be the case. When the wild duck 
became domesticated, and lived in a farm-yard, all the individuals 
were compelled to walk and stand more than they had done 
previously, and the muscles of the legs were used to a correspond- 
ingly greater degree. The same thing happens in the wild state, 
when any change in the conditions of life compels an organ to be 
more largely used. No individual will be able to entirely avoid 
this extra use, and each will endeavour to accommodate itself to 
the new conditions according to its power. The amount of this 
power depends upon the predisposition of the germ ; and natural 
selection, while it apparently decides between individuals of various 
degrees of strength, is in truth operating upon the stronger and 
weaker germs. 

But the very conclusions which have been drawn from the 
increase of activity which has arisen from exercise, must also be 
drawn from the instances of atrophy or degeneration follo\ving from 
the disuse of organs. 

Darwin long ago called attention to the fact that the degeneration 
of an organ may, under certain circumst^ances, be beneficial to the 
species. For example, he first proved in the instance of Madeira, 
that the loss of wings may be of advantage to many beetles inhabit- 
ing oceanic islands. The individuals with imperfectly developed or 
atrophied wings have an advantage, because they are not carried 
out to sea by the frequent winds. The small eyes, buried in fur, 
possessed by moles and other subterranean mammals, can be similarly 
explained by means of natural selection. So also, the complete dis- 
appearance of the limbs of snakes is evidently a real advantage to 
animals which creep through narrow holes and clefts ; and the de- 
generation of the wings in the ostrich and penguin is, in part, 
explicable as a favourable modification of the organ of flight into 
an organ for striking air or water respectively. 

But when the degeneration of disused organs confers no benefits 
upon the individual, the explanation becomes less simple. Thus 
we find that the eyes of animals which inhabit dark caves (such 
as insects, crabs, fish, Amphibia, etc.) have undergone degeneration ; 
yet this can hardly be of direct advantage to the animals, for they 
could live quite as well in the dark with well-developed eyes. But 
we are here brought into contact with a very important aspect of 



86 ON HEREDITY. 

'natural selection, viz. the power of conservation exerted by it. 
Not only does the survival of the fitt est select the hest. but it als o . 
j naintains it ^. The struggle for existence does not cease with the 
foundation of a new specific type, or with some perfect adaptation 
to the external or internal conditions of life, but it becomes, on the 
contrary, even more severe, so that the most minute differences of 
structure determine the issue between life and death. 

The sharpest sight possessed by birds is found in birds of prey, 
but if one of them entered the world with eyes rather below 
the average in this respect, it could not, in the long run, escape 
death from hunger, because it would always be at a disadvantage as 
compared with othera. 

Hence the sharp sight of these birds is maintained by means of 
I the continued operation of natural selection, by which the indi- 
viduals with the weakest sight are being continually exterminated. 
But all this would be changed at once, if a bird of prey of a certain 
species were compelled to live in absolute darkness. The quality of 
the eyes would then be immaterial, for it could make no difference 
to the existence of the individual, or the maintenance of the species. 
The sharp sight might, perhaps, be transmitted through numerous 
generations ; but when weaker eyes arose from time to time, these 
would also be transmitted, for even very short-sighted or imperfect 
eyes would bring no disadvantage to their owner. Hence, by con- 
tinual crossing between individuals with the most varied degrees 
of perfection in this respect, the average of perfection would gradu- 
ally decline from the point attained before the species lived in the 
dark. 

We do not at present know of any bird living in perfect 
darkness, and it is improbable that such a bird will ever be found ; 
but we are acquainted with blind fish and Amphibia, and among 
these the eyes are present it is true, but they are small and hidden 
under the skin. I think it is difficult to reconcile the facts of the 
case with the ordinary theory that the eyes of these animals have 
simply degenerated through disuse. If disuse were able to bring 
about the complete atrophy of an organ, it follows that every trace 
of it would be effaced. We know that, as a matter of fact, the 
olfactory organ of the frog completely degenerates when the olfactory 

* This principle was, I believe, first pointed out by Seidlitz. Compare Seidlitz, 
* Die Darwin'sche Theorie,' Leipzig, 1875, p. 198. 



ON HEREDITY. 87 

nerve is divided ; and that great degeneration of the eye may be 
brought about by the artificial desti-uction of the optic centre in 
the brain. Since, therefore, the effects of disuse are so striking in 
a single life, we should certainly expect, if such effects can be trans- 
mitted, that all traces of an eye would soon disappear from a species 
which lives in the dark. 

The caverns in Carniola and Carinthia, in which the blind Proteus 
and so many other blind animals liv», belong geologically to the 
Jurassic formation ; and although we do not exactly know when for 
example the Protens first entered them, the low organization of this 
amphibian certainly indicates that it has been sheltered there for 
a very long period of time, and that thousands of generations of 
this species have succeeded one another in the caves. 

Hence there is no reason to wonder at the extent to which the 
degeneration of the eye has been already carried in the Proteiis ; 
even if we assume that it is merely due to the cessation of the 
conserving influence of natural selection." 

But it is unnecessary to depend upon this assumption alone, for 
when a useless organ degenerates, there are also other factors which 
demand consideration, namely, the higher development of other 
organs which compensate for the loss of the degenerating structure, \ 
or the increase in size of adjacent parts. If these newer develop- 
ments are of advantage to the species, they finally come to take 
the place of the organ which natural selection has failed to 
preserve at its point of highest perfection. 

In the first place, a certain form of correlation, which Roux^ 
calls ' the struggle of the parts in the organism,' plays a most 
important part. Cases of atrophy, following disuse, appear to be 
always attended by a corresponding increase of other organs : blind 
animals always possess very strongly developed organs of touch, 
hearing, and smell, and the degeneration of the wing-muscles of 
the ostrich is accompanied by a great increase in the strength of 
the muscles of the leg. If the average amount of food which an 
animal can assimilate every day remains constant for a considerable 
time, it follows that a strong influx towards one organ must be 
accompanied by a drain upon others, and this tendency will increase, 
from generation to generation, in proportion to the development of 

^ W. Enux, * Der Kampf der Theile im Organismus,' Leipzig, 1881. 



88 ON HEREDITY. 

the growing- organ, which is favoured by natural selection in its 
increased blood-supply, etc. ; while the operation of natural selection 
has also determined the organ which can bear a corresponding loss 
without detriment to the organism as a whole. 

"Without the operation of natural selection upon different indi- 
viduals, the struggle between the organs of a single individual 
would be unable to encourage a predisposition in the germ towards 
the degeneration or non-development of a useless organ, and it 
could only limit and degrade the development of an organ in the 
lifetime of the individual. / If, therefore, acquired characters are not 
transmitted, the disposition to develope such an organ would be 
present in the same degree in each successive generation, although 
the realization would be less perfect. The complete disappearance 
of a rudimentary organ can only take place by the operation of 
natural selection ; this principle will lead to its elimination, inas- 
much as the disappearing structure takes the place and the nutriment 
of other useful and important organs. Hence the process of natural 
selection tends to entirely remove the former. The predisposition 
towards a weaker development of the organ is thus advantageous, 
and there is every reason for the belief that the advantages would 
continue to be gained, and that therefore the processes of natural 
selection would remain in operation, until the germ had entirely 
lost all tendency towards the development of the organ in question. 
The extreme slowness with which this process takes place, and the 
extraordinary persistence of rudimentary organs, at any rate in 
the embryo, together with their gradual but finally complete dis- 
appearance, can be clearly seen in the limbs of certain vertebrates 
and arthropods. The blind-worms have no limbs, but a rudi- 
mentary shoulder-girdle is present close under the skin, and the 
interesting fact has been quite recently established ^ that the fore- 
limbs are present in the embryo in the form of short stumps, which 
entirely disappear at a later stage. In most snakes all traces of 
limbs have been lost in the adult, but we do not yet know for 
certain whether they are also wanting in the embryo. I might 
further mention the very different stages of degeneration witnessed 
in the limbs of various salamanders ; and the anterior limbs of 
Hegperornis — the remarkable toothed bird from the cretaceous rocks 

* Compare Bom in 'Zoolog. Anzeiger,' 1883, No. 150, p. 537. 



ON HEREDITY. 89 

' — which, according to Marsh ^, consists only of a very thin and 
relatively small humerus, which was probably concealed beneath the 
skin. The water-fleas [Baplmidae) possess in the embryonic state 
three complete and almost equal pairs of jaws, but two of these 
entirely disappear, and do not develope into jaws in any species. 
In the same way, the embryo of the magg-ot-like legless larva of 
bees and wasps possesses three pairs of ancestral limbs. 

There are, however, cases in which, apparently, acquired variations 
of characters are transmitted without natural selection playing any 
active part in the change. Such a case is afforded by the sliort- 
sightedness so common in civilized nations. 

This affection is certainly hereditary in some cases, and it may 
well have been explained as an example of the transmission of 
acquired changes. It has been argued, that acquired short-sighted- 
ness can be in a slight degree transmitted, and that each successive 
generation has developed a further degree of the disease by habitu- 
ally holding books etc. close to the eyes, so that the inborn pre- 
disposition to short-sightedness is continually accumulating. 

But we must remember that variations in the refraction of the 
human eye have been for a long time independent of the pre- 
serving control of natural selection. In the struggle for existence, 
a blind man would certainly disappear before those endowed with 
sight, but myopia does not prevent any one from gaining a living. 

A short-sighted lynx, hawk, or gazelle, or even a short-sighted 
Indian, would be eliminated by naturiil selection, but a short-sighted 
European of the higher class finds no difficulty in earning his 
bread. 

Those fluctuations on either side of the average which we call 
myopia and hypermetropia, occm* in the same manner, and are due to 
the same causes, as those which operate in producing degeneration in 
the eyes of cave-dwelling animals. If, therefore, we not infrequent! / 
meet with families in which myopia is hereditary, such results m; > 
be attributed to the transmission of an accidental disposition on t liv> 
part of the germ, instead of to the transmission of acquired short- 
sightedness, A very large proportion of short-sighted people do 
not owe their affliction to inheritance at all, but have acquired it 
for themselves ; for there is no doubt that a normal eye may be 

* 0. C. Marsh, ' Odontornithes, a Monograph on the extinct toothed Bu-ds of 
North America,' Washington, 1880. 



90 ON HEREDITY. 

rendered myopic in the course of a life-time by continually looking 
at objects from a very short distance, even when no hereditary 
predisposition towards the disease can be shown to exist. Such a 
change would of course appear more readily if there was also a 
corresponding predisposition on the part of the eye. But I should 
not explain this widely spread predisposition towards myopia as 
due to the transmission of acquired short-sightedness, but to the 
greater variability of the eye, which necessarily results from the 
cessation of the controlling influence of natural selection. 

This suspension of the preserving influence of natural selection 
may be termed PanmixiaJ for all individuals can reproduce them- 
selves and thus stamp their characters upon the species, and not 
only those which are in all respects, or in respect to some single organ, 
the fittest. In my opinion, the greater number of those variations 
which are usually attributed to the direct influence of external 
conditions of life, are to be ascribed to panmixia. For example, the 
great variability of most domesticated animals essentially depends 
upon this principle. 

A goose or a duck must possess strong powers of flight in the 
natural state, but such powers are no longer necessary for obtaining 
food when it is brought into the poultry-yard, so that a rigid selec- 
tion of individuals with well-developed wings, at once ceases among 
its descendants. Hence in the course of generations, a deterioration 
of the organs of flight must necessarily ensue, and the other 
members and organs of the bifd will be similarly affected. 

This example very clearly indicates that the degeneration of an 
organ does not depend upon its disuse ; for although our domestic 
poultry very rarely make use of their wings, the muscles of flight 
have not disappeared, and, at any rate in the goose, do not seem 
to have undergone any marked degeneration. 

The numerous and exact observations conducted by Darwin 
upon the weight and measurement of the bones in domestic fowls, 
seem to me to possess a significance beyond that which he attributed 
to them. 

If the weight of the wing-bones of the domestic duck bears 
a smaller proportion to the weight of the leg-bones than in the 
wild duck, and if, as Darwin rightly assumes, this depends not 
only upon the diminution of the wings, but also upon the increase 
of the legs, it by no means follows that this latter increase in 



ON HEREDITY. 91 

organs which are now more frequently used, is dependent upon 
hereditary influences alone. 

It is quite possible that it depends, on the one hand, upon the 
suspension of natural selection, or panmixia (and these effects would 
be transmitted), and on the other hand upon the direct influence 
of increased use during the course of a single life. We do not 
yet know with any accuracy, the amount of change which may be 
produced by increased use in the course of a single life. If it is 
desired to prove that use and disuse produce hereditary effects 
without the assistance of natural selection, it will be necessary to 
domesticate wild animals (for example the wild duck) and preserve 
all their descendants, thus excluding the operation of natural selec- 
tion. If then all individuals of the second, third, fourth and later 
generations of these tame ducks possess identical variations, which 
increase from generation to generation, and if the nature of these 
changes proves that they must have been due to the effect of use 
or disuse, then perhaps the transmission of such effects may be- 
admitted ; but it must always be remembered that domestication 
itself influences the organism, — not only directly, but also indirectly, 
by the increase of variability as a result of the suspension of natural 
selection. Such experiments have not yet been carried out in 
sufficient detail ^. 

It is usually considered that the origin and variation of instincts 
are also dependent upon the exercise of certain groups of muscles and 
nerves during a single Hfe-time ; and that the gradual improve- 
ment which is thus caused by practice, is accumulated by hereditary 
transmission. I believe that this is an entirely erroneous view, 
and I hold that all instinct is entirely due to the operation of natural 
selection, and has its foundation, not upon inherited experiences, 
but upon the variations of the germ. 

Why, for instance, should not the instinct to fly from enemies 
have arisen by the survival of those individuals which are naturally 
timid and easily startled, together with the extermination of those 
which are unwary ? It may be urged in opposition to this explana- 
tion that the birds of uninhabited islands which are not at first 
shy of man, acquire in a few generations an instinctive dread of. 
him, an instinct which cannot have arisen in so short a time 

* C. Darwin, 'Variation of Animals and Plants under Domestication.' Vol. I. 



92 ON HEREDITY. 

by means of natural selection. But in this case are we really 
dealing with the orig'in of a new instinct, or only with the addition 
of one new perception (' Wahrnehmung,' Schneider) ^, of the same 
kind as those which incite to the instinct of flight — an instinct 
which had been previously developed in past ages but had never 
been called forth by man ? Again, has any one ascertained whether 
the young birds of the second or third generation are frightened 
by man ? May it not be that the experience of a single life-time 
plays a great part in the origin of the habit ? For my part, I am 
inclined to believe that the habit of flying from man is developed 
in the first generation which encounters him as a foe^. We see 
how wary and cautious a flock of birds become as soon as a few 
shots have been fired at them, and yet shortly before this occur- 
rence they were perhaps playing carelessly close to the sportsmen. 
Intelligence plays a considerable part in the life of birds, and 
it by no means follows that the transmission of individual habits 
explains the above-mentioned phenomena. The long-continued 
operation of natural selection may veiy well have been necessary 
before the perception of man could awake the instinct to flee in 
young, inexperienced birds. Unfortunately the observations upon 
these points are far too indefinite to enable us to draw conclusions. 
There is again the frequently -quoted instance of the young 
pointer, ' which, untrained, and without any example which might 
have been imitated, pointed at a lizard in a subtropical jungle, just 
as many of its forefathers had pointed at partridges on the plain of 
St. Denis,' and which, without knowing the effect of a shot, sprang 
forward barking, at the first discharge, to bring in the game. This 
conduct must not be attributed to the inheritance of any mental 
picture, such as the effect of a shot, but to the inheritance of a 
certain reflex mechanism. The young pointer does not spring 
forward at the shot because he has inherited from his forefathers a 

^ Compare ' Der thierische Wille,' Leipzig, 1880. 

' Steller's interesting account of the Sea-cow {Rhytina Stelleri) proves that this 
suggestion is valid. This large mammal was living in great numbers in Behring 
Strait at the end of the last century, but has since been entirely exterminated by 
man. Steller, who was compelled by shipwreck to remain in the locality for a whole 
year, tells us that the animals were at first without any fear of man, so that they 
could be approached in boats and could thus be killed. After a few months how- 
ever the survivors became wary, and did not allow Steller's men to approach them, 
so that they were difficult to catch. — A. W., 1888. 



ON HEREDITY. . 93 

certain association of ideas, — shot and game, — but because he has 
inherited a reflex mechanism, which impels him to start forward 
on hearing" a report. We cannot yet determine without more ex- 
periments how such an impulse due to perception (' Wahrnehmung-- 
strieb,' Schneider) has arisen ; but, in my opinion, it is almost in- 
conceivable that artificial breeding- has had nothing* to do with it ; 
and that we are here concerned — not with the inheritance of the 
effects of training — but with some pre-disposition on the part of the 
germ, which has been increased by artificial selection. 

The necessity for extreme caution in appealing to the supposed 
hereditary effects of use, is well shown in the case of those 
numerous instincts, which only come into play once in a life- 
time, and which do not therefore admit of improvement by practice. 
The queen-bee takes her nuptial flight only once, and yet how 
many and complex are the instincts and the reflex mechanisms 
which come into play on that occasion. Again, in many insects 
the deposition of eggs occurs but once in a life-time, and yet 
such insects always fulfil the necessary conditions with unfailing 
accuracy, either simply dropping the eggs into water, or carefully 
fixing them on the surface of the earth beneath some stone, or 
laying them on a particular part of a certain species of plant ; and 
in all these cases the most complicated actions are performed. It 
is indeed astonishing to watch one of the Cynipidae [Rhodites 
Tosae) depositing her eggs in the tissue of a young bud. She first 
carefully examines the bud on all sides, and feels it with her legs 
and antennae. Then she slowly inserts her long ovipositor between 
the closely-rolled leaves of the bud, but if it does not reach exactly 
the right spot, she will withdraw and re-insert it many times, until 
at length, when the proper place has been found, she will slowly 
bore deep into the very centre of the bud, so that the egg will 
reach the exact spot, where the necessary conditions for its develop- 
ment alone exist. 

But each Cynips lays eggs many times, and it may be argued that 
practice may have led to improvement in this case ; we cannot 
however, as a matter of fact, expect much improvement in a process 
which is repeated, perhaps a dozen times, at short intervals of 
time, and which is of such an excessively complex nature. 

It is the same with the deposition of eggs in most insects. How 
can practice have had any influence upon the origin of the instinct 



94 . ON HEREDITY. 

which leads one of our butterflies — ( Vanessa levana) — to lay its green 
eggs in single file, as columns, which project freely from the stem 
or leaf, so that protection is gained by their close resemblance to the 
flower-buds of the stinging-nettle, which forms the food-plant of 
their caterpillars? 

Of course the butterfly is not aware of the advantage which 
follows from such a proceeding ; intelligence has no part in the 
process. The entire operation depends upon certain inherent, ana- 
tomical and physiological arrangements: — on the structure of the 
ovary and oviducts, on the simultaneous ripening of a certain 
number of eggs, and on certain very complex reflex mechanisms 
which compel the butterfly to lay its eggs on certain parts of 
certain plants. Schneider is certainly right when te maintains that 
this mechanism is released by a sensation, arising from the percep- 
tion (whether by sight or smell, or both together) of the particular 
plant or part of the plant upon which the eggs are to be laid ^. 
At any rate, we cannot, in such cases, appeal to the effects of 
constant use and the transmission of acquired characters, as an 
explanation ; and the origin of the impulse can only be understood 
as a result of the process of natural selection. 

The protective cocoons by which the pupae of many insects are 
surrounded also belong to the same category, and improvement by 
practice is entirely out of the question, for they are only constructed 
once in the course of a life-time. And yet these cocoons are often 
remarkably complex : think, for instance, of the cocoon spim by the 
caterpillar of the emperor moth (Sattirnia carpini), which is so 
tough that it can hardly be torn, and which the moth would be 
unable to leave, if an opening were not provided for the purpose ; 
while, on the other hand, the pupa would not be defended against 
enemies if the opening were not furnished with a circle of pointed 
bristles, converging outwards, on the principle of the lobster pot, so 
that the moth can easily emerge, although no enemy can enter. 
The impulse which leads to the production of such a structure can 
only have arisen by the operation of natural selection — not, of 
course, during the history of a single species, but during the de- 
velopment of numerous, consecutive species — by gradual and un- 
ceasing improvements in the initial stages of cocoon-building. 

* Compare Schneider, ' Der thierische Wille.' 



ON HEREDITY. • . 95 

A number of species exists at the present day, of which the cocoons 
can be arranged in a complete series, becoming- gradually less and 
less complex, from that described above, down to a loosely-con- 
structed, spherical case in which the pupa is contained. 

The cocoon spun by the larva of Saturnia carpini differs but little 
in complexity from the web of the spider, and if the former is con- 
structed without assistance from the experience of the single 
individual — and this must certainl}^ be admitted — it follows that the 
latter may be also built without the aid of experience, while there is 
neither reason nor necessity for appealing to the entirely unproved 
transmission of acquired skill in order to explain this and a thousand 
other operations. 

It may be objected that, in man, in addition to the instincts 
inherent in every individual, special individual predispositions are 
also found, of such a nature that it is impossible that they can 
have arisen by individual variations of the germ. On the other 
hand, these predispositions — which we call talents — cannot have 
arisen through natural selection, because life is in no way dependent 
upon their presence, and there seems to be no way of explaining 
their origin except by an assumption of the summation of the skill 
attained by exercise in the course of each single life. In this case, 
therefore, we seem at first sight to be compelled to accept the 
transmission of acquired characters. 

Now it cannot be denied that all predispositions may be improved 
by practice dm-ing the course of a life-time, — and, in truth, very 
remarkably improved. If we could explain the existence of great 
talent, such as, for example, a gift for music, painting, sculpture, or 
mathematics, as due to the presence or absence of a special organ in 
the brain, it follows that we could only understand its origin and 
increase (natural selection being excluded) by accumulation, due to 
thq transmission of the results of practice through a series of 
generations. But talents are not dependent upon the possession of 
special organs in the brain. They are not simple mental dis- 
J)Ositions, but combinations of many dispositions, and often of 
a most complex nature : they depend upon a certain degree of 
irritability, and a power of readily transmitting impulses along the 
nerve-tracts of the brain, as well as upon the especial development 
of single parts of the brain. In my opinion, there is absolutely no 
trustworthy proof that talents have been improved by their exercise 



96 ON HEKEDITY. 

through the course of a long series of generations. The Bach 
family shows that musical talent, and the Bernoulli family that 
mathematical power, can be transmitted from generation to genera- 
tion, but this teaches us nothing as to the origin of such talents. 
In both families the high-water mark of talent lies, not at the end 
of the series of generations, as it should do if the results of practice 
are transmitted, but in the middle. Again, talents frequently 
appear in some single member of a family which has not been 
previously distinguished. 

Gauss was not the son of a mathematician ; Handel's father was 
a surgeon, of whose musical powers nothing is known ; Titian was 
the son and also the nephew of a lawyer, while he and his brother, 
Francesco Vecellio, were the first painters in a family which pro- 
duced a succession of seven other artists with diminishing talents. 
These facts do not, however, prove that the condition of the nerve- 
tracts and centres of the brain, which determine the specific talent, 
appeared for the first time in these men : the appropriate condition 
surely existed previously in their parents, although it did not 
achieve expression. They prove, as it seems to me, that a high 
degree of endowment in a special direction, which we call talent, 
cannot have arisen from the experience of previous generations, that 
is, by the exercise of the brain in the same specific direction. 

It appears to me that talent consists in a happy combination of 
exceptionally high gifts, developed in one special direction. At 
present, it is of course impossible to understand the physiological 
conditions which render the origin of such combinations possible, 
but it is very probable that the crossing of the mental dispositions 
of the parents plays a great part in it. This has been admirably 
and concisely expressed by Goethe in describing his own charac- 
teristics — 

Vom Vater hab' icli die Statur 
Des Lebens emstes Ffihren, 
Vom Miitterchen die Frohnatur 
Die Lust zum Fabuliren, etc. 

The combination of talents frequently found in one individual, 
and the appearance of different remarkable talents in the various 
branches of one and the same family, indicate that talents are only 
special combinations of certain highly-developed mental dispositions 
which are found in every brain. Many painters have been admir- 
able musicians, and we very frequently find both these talents 



ON HEREDITY. 97 

developed to a slighter extent in a sing-le individual. In the 
Feuerbach family we find a distinguished jurist, a remarkable philo- 
sopher, and a highly-talented artist ; and among the Mendelssohns 
a philosopher as well as a musician. 

The sudden and yet widespread appearance of a particular talent 
in correspondence with the general intellectual excitement of a 
certain epoch points in the same direction. How many poets arose 
in Germany during the period of sentiment which marked the 
close of the last century, and how completely all poetic gifts seem 
to have disappeared during the Thirty Years' War. How numerous 
were the philosophers that appeared in the epoch which succeeded 
Kant ; while all philosophic talent seemed to have deserted the 
German nation during the sway of the antagonistic ' exact science,' 
with its contempt for speculation. 

Wherever academies are founded, there the Schwanthalers, 
Defreggers, and Lenbachs emerge from the masses which had 
shown no sign of artistic endowment through long periods of time ^. 
At the present day there are many men of science who, had they 
lived at the time of Biirger, Uhland, or Schelling, would probably 
have been poets or philosophers. And the man of science also can- 
not dispense with that mental disposition directed in a certain course, 
which we call talent, although the specific part of it may not be 
so obvious : we may, indeed, go further, and maintain that the 
Physicist and the Chemist are characterized by a combination of 
mental dispositions which diflfer from those of the Botanist and the 
Zoologist. Nevertheless, a man is not born a physicist or a 
botanist, and in most cases chance alone determines whether his 
endowments are developed in either direction. 

Lessing has asked whether Raphael would have been a less 
distinguished artist had he been born without hands: we might 
also enquire whether he might not have been as great a musician 
as he was painter if, instead of living during the historical high- 
water mark of painting, he had lived, under favourable personal 
influences, at the time of highly-developed and widespread musical 
genius. A great artist is always a great man, and if he finds the 
outlet for his talent closed on one side, he forces his way through 
on the other. 

From all these examples I wish to show that, in my opinion, 

[' The author refers to the Academy of Arts at Munich. S. S.] 
H 



98 ON HEREDITY. 

* 

talents do not appear to depend upon the improvement of any 

special mental quality by continued practice, but they are the 
expression, and to a certain extent the bye-product, of the human 
mind, which is so highly developed in all directions. 

But if any one asks whether this high mental development, 
acquired in the course of innumerable generations of men, is not 
dependent upon the hereditary effects of use, I would remind him 
that human intelligence in general is the chief means and the 
chief weapon which has served and still serves the human species 
in the struggle for existence ^. . Even in the present state of 
civilization — distorted as it is by numerous artificial encroachments 
and unnatural conditions — the degree of intelligence possessed by 
the individual chiefly decides between destruction and life ; and 
in a natm*al state, or still better in a state of low civilization, this 
result is even more striking. 

Here again, therefore, we encounter the effects of natural selection, 
and to this power we must attribute, at any rate, a great part of 
the phenomena we have been discussing, and it cannot be shown 
that — in addition to its operation — the transmission of characters 
acquired by practice plays any part in nature. 

I only know of one class of changes in the organism which is 
with difficulty explained by the supposition of changes in the 
germ ; these are the modifications which appear as the direct 
consequence of some alteration in the surroundings. But our 
knowledge on this subject is still very defective, and we do not 
know the facts with sufficient precision to enable us to pronounce 
a final verdict as to the cause of such changes : and for this reason, 
I do not propose to consider the subject in detail. 

These changes — such, for example, as are produced by a strange 
climate — ^have been always looked at under the supposition that 
they are transmitted and intensified from generation to generation, 
and for this reason the observations are not always sufficiently 
precise. It is difficult to say whether the changed climate may 
not have first changed the germ, and if this were the case the 
accumulation of effects through the action of heredity would pre- 
sent no difficulty. For instance^ it is well known that increased 
nourishment not only causes a plant to grow more luxuriantly, 
but it alters the plant in some distinct way, and it would be 

' Compare Darwin's ' Descent of Man.' 



ON HEREDITY. 99 

wonderful indeed if the seeds were not also larger and better furnished 
with nutritive material. If the increased nourishment be repeated 
in the next generation, a still farther increase in the size of the 
seed, in the luxuriance of the plant, and in all other changes which 
ensue, is at any rate conceivable if it is not a necessity. But this 
would not be an instance of the transmission of acquired characters, 
but only the consequence of a direct influence upon the germ-cells, 
and of better nourishment during growth. 

A similar interpretation explains the converse change. When^ 
horses of normal size are introduced into the Falkland Islands, 
the next generation is smaller in consequence of poor nourishment 
and the damp climate, and after a few generations they have de- 
teriorated to a marked extent. In such a case we have only to 
assume that the climate which is unfavourable and the nutriment 
which is insufficient for horses, afi^ect not only the animal as a 
whole, but also its germ-cells. This would result in the diminution 
in size of the germ-cells, the effects upon, the offspring being still 
further intensified by the insuflfieient nourishment supplied during 
growth. 'But such results would not depend upon the transmission 
by the germ-cells of certain peculiarities due to the unfavourable 
climate, which only appear in the full-grown horse. 

It must be admitted that there are cases, such as the climatic 
varieties of certain butterflies, which raise some difficulties against 
this explanation. I myself, some years ago, experimentally investi- 
gated one such case ^, and even now I cannot explain the facts 
otherwise than by supposing the passive acquisition of characters 
produced by the direct influence of climate. 

It must be remembered, however, that my experiments, which 
have been repeated upon several American species by H. W. 
Edwards, with results confirmatory of my own in all essential 
respects, were not undertaken with the object of investigating the 
question from this point of view alone. New experiments, under 
varying conditions, will be necessary to afford the true explana- 
tion of this aspect of the question ; and I have already begun to 
undertake them. 

Leaving on one side, for the moment, these doubtful, and 

^ ' Studien zur Descendenztheorie, I. TJeber den Saison-Dimorphismus der 
Schmetterlinge.' Leipzig, 1875. English edition translated and edited by Professor 
Meldola, ' Studies in the Theory of Descent,' Part I. 

H % 



100 ON HEREDITY. 

insufficiently investigated cases, we may still maintain that the 
assumption that changes induced by external conditions in the 
organism as a whole, are communicated to the germ-cells after the 
manner indicated in Darwin's hypothesis of pangenesis, — is wholly 
unnecessary for the explanation of these phenomena. Still we 
cannot exclude the possibility of such a transmission occasionally 
occurring, for, even if the greater part of the effects must be attri- 
buted to natural selection, there might be a smaller part in certain 
cases which depends on this exceptional factor. 

A complete and satisfactory refutation of such an opinion cannot 
be brought forward at present : we can only point out that such 
an assumption introduces new and entirely obscure forces, and that 
innumerable cases exist in which we can certainly exclude all 
assistance from the transmission of acquired characters. In most 
cases of variation in colour we have no explanation but the survival 
of the fittest ^, and the same holds good for all changes of form 
which cannot be influenced by the will of the animal. Very 
numerous adaptations, such, for instance, as occur in the eggs of 
animals, — the markings, and appendages which conceal them from 
enemies, the complex coverings which prevent them from drying 
up or protect them from the injurious influence of eold,^must 
have all arisen entirely independently of any expression of will, 
or of any conscious or unconscious action on the part of the 
animals. I will not mention here the case of plants, which as 
every one knows are unconscious, for they are beyond my province. 
In this matter, there can be no suggestion of adaptation depending 
upon a struggle between the various pai-ts of the organism (Roux)^. 
Natural selection cannot operate upon the different epithelial cells 
which secrete the egg-shell of Apus, since it is of no consequence to 
the animal which secretes the egg-shell whether a good or a bad 
shell is produced. Natural selection first operates among the off- 
spring, and the eg^ with a shell incapable of resisting cold or 
droiight is destroyed. The different cells of the same individual 
are not selected, but the different individuals themselves. 

In all such cases we have no explanation except the operation 
of natural selection, and if we cannot accept this, we may as well 

' The colours which have been called forth by sexual selection must also be in- 
cluded here. 
* Wilhelin Roux, ' Der Kampf der Theile im Organismus.' Leipzig, i8Si. 



ON HEREDITY. 101 

abandon any attempt at a natural explanation. But, in my 
opinion, there is no reason why natural selection should be con- 
sidered inadequate to the task. It is true that the objection has 
been lately ui-g-ed, that it is inconceivable that all the wonderful 
adaptations of the organism to its sun-ounding-s can have arisen 
through the selection of individuals ; and that for this purpose an 
infinite number of individuals and infinite time would be required ; 
and stress is laid upon the fact that the wished-for useful changes 
can only arise singly and very rarely among a great number 
of individuals. 

This last objection to the modern conception of natural selec- 
tion has apparently some weight, for, as a matter of fact, useful 
variations of a conspicuous kind seldom appear, and are often 
entirely absent for many generations. If we expect to find 
that qualitative changes take place by sudden leaps, we can 
never escape this difficulty. But, I think, we must not look 
for conspicuous variations — such as occur among domesticated 
animals and plants — in the process of the evolution of species 
as it goes on in nature. Natural selection does not deal with 
qualitative but quantitative changes in the individual, and the 
latter are always present. 

A simple example will make this clearer. Let us suppose that 
it was advantageous to some species — for instance the ancestors of 
the giraffe — to lengthen some part of the body, such as the neck : 
this result could be obtained in a relatively short time, for the 
members of the species already possessed necks of varying length, 
and the variations which form the material for natural selection 
were already in existence. Now all the organs of every species 
vary in size, and any one of them wall undergo constant and 
progressive increase, as soon as it acquires exceptional usefulness. 
But not only will the organ fluctuate as a whole, but also the 
parts composing it will become larger or smaller under given con- 
ditions, will increase or diminish by the operation of natural selec- 
tion. I believe that qualitative variations always depend upon 
difl'erences in the size and number of the component parts of 
the whole. A skin appears to be naked, when it is really covered 
with a number of small fine hairs : if these grow larger and increase 
in number, a thick covering is formed, and we say that the skin 
is woolly or furry. In the same way the skin of many worms and 



102 ON HEREDITY. 

Crustacea is apparently colourless, but the microscope reveals the 
presence of a number of beautiful pig-ment spots ; and not until 
these have increased enormously does the skin appear coloured 
to the naked eye. The presence or absence of colour and its 
quality when present are here dependent upon the quantity of 
the most minute particles, and on the distance at which the 
object in question is observed. Again, the first appearance of 
colour, or the change from a green to a yellow or red colour 
depends upon slight variations in the position or in the number 
of the oxygen atoms which enter into the chemical combination 
in question. Fluctuations in the chemical composition of the mole- 
cules of a unicellular organism (for example) must continually 
arise, just as fluctuations are always occurring in the number of 
pigment granules in a certain cell, or in the number of pigment 
cells in a certain region of the body, or even in the size of the 
various parts of the body. 

All these quantitative relations are exposed to individual fluctua- 
tions in every species ; and natural selection can strengthen the 
fluctuations of any paxt, and thus cause it to develope further in 
any given direction. 

From this point of view, it becomes less astonishing and less 
inconceivable that organisms adapt themselves — as we see that they 
obviously do — in all their parts to any condition of existence, and 
that they behave like a plastic mass which can be moulded into 
almost any imaginable form in the course of time. 

If we ask in what lies the cause of this variability, the answer 
must undoubtedly be that it lies in the germ-cells. From the 
moment when the phenomena which precede segmentation com- 
mence in the egg, the exact kind of organism which will be 
developed is already determined — whether it will be larger or 
smaller, more like its father or its mother, which of its parts will 
resemble the one and which the other, even to the minutest detail. 
In spite of this, there still remains a certain scope for the influence 
of external conditions upon the organism. But this scope is 
limited, and forms but a small area round the fixed central point 
which is determined by heredity. Abundant nourishment can 
make the body large and strong, but can never make a giant out 
of the germ -cell destined to become a dwarf. Unhealthy seden- 
tary habits or insufiicient nourishment makes the factory-hand pale 



ON HEREDITY. 103 

and stunted ; life on board ship, with plenty of exercise and sea 
air, gives the sailor bodily strength and a tanned skin ; but when 
once the resemblance to father or mother, or to both, is established 
in the germ-cell it can never be effaced, let the habit of life be 
what it will. 

But if the essential nature of the germ-cell dominates over the 
organism which will grow from it, so also the quantitative in- 
dividual differences, to which I referred just now, are, by the same 
principle, established in the germ, and — whatever be the cause 
which determines their presence — they must be looked upon as 
inherent in it. It therefore follows that, although natural selection 
appears to operate upon the qualities of the developed organism 
alone, it in tmth works upon peculiarities which lie hidden in the 
germ-cells. Just as the final development of any predisposition 
in the germ, and just as any character in the mature organism 
vibrates with a certain amplitude around a fixed central point, 
so the predisposition of the germ itself fluctuates, and it is on 
this that the possibility of an increase of the predisposition in 
question, and its average result, depends. 

If we trace all the permanent hereditary variations from 
generation to generation back to the quantitative variations 
of the germ, as I have sought to do, the question naturally 
occurs as to the source from which these variations arose in 
the germ itself. I will not enter into this subject at any length 
on the present occasion, for I have already expressed my opinion 
upon it^. 

I believe however that they can be referred to the various ex- 
ternal influences to which the germ is exposed before the com- 
mencement of embryonic development. Hence we may fairly 
attribute to the adult organism influences which determine the 
phyletic development of its descendants. For the germ-cells are 
contained in the organism, and the external influences which affect 
them are intimately connected with the state of the organism in 
which they lie hid. If it be well nourished, the germ-cells will 
have abundant nutriment ; and, conversely, if it be weak and 
sickly, the germ-cells will be arrested in their growth. It is even 

^ Consult ' Studien zur Descendenztheorie, IV. tJber die meclianische AufiFassung 
der Natur,' p. 303, etc. Translated and edited by Professor Meldola; see 'Studies 
in the Theory of Descent,' p. 677, &c. 



104 ON HEREDITY. 

possible that the effects of these influences may be more specialized ; 
that is to say, they may act only upon certain parts of the germ- 
cells. But this is indeed very different from believing- that the 
changes of the organism which result from external stimuli can 
be transmitted to the germ-colls and will re-develope in the next 
generation at the same time as that at which they arose in the 
parent, and in the same part of the organism. 

" We have an obvious means by which the inheritance of all 
transmitted peculiarities takes place, in the contimdty of the 
substance of the germ-celk, or germ-plasm. If, as I believe, the 
substance of the germ-cells, the germ-plasm, has remained in per- 
petual continuity from the first origin of life, and if the germ- 
plasm and the substance of the body, the somatoplasm, have always 
occupied different spheres, and if changes in the latter only arise 
when they have been preceded by corresponding changes in the 
former, then we can, up to a certain point, understand the principle 

^of heredity; or, at any rate, we can conceive that the human 
mind may at some time be capable of understanding it. We may 
at least maintain that it has been rendered intelligible, for we 
can thus trace heredity back to growth ; we can thus look upon 
reproduction as an overgrowth of the individual, and can thus 
distinguish between a succession of species and a succession of 
individuals, because in the latter succession the germ-plasm remains 
similar, while in the succession of the former it becomes different. 
Thus individuals, as they arise, ai'e always assuming new and more 
complex forms, until the interval between the simple unicellular 
protozoon and the most complex of all organisms — man himself — 
is bridged over. 

I have not been able to throw light upon all sides of the question 
which we are here discussing. There are still some essential points 
which I must leave for the present ; and, furthermore, I am not 
yet in a position to explain satisfactorily all the details which 
arise at every step of the argument. But it appeared to me to be 
necessary to state this weighty and fundamental question, and to 
formulate it concisely and definitely ; for only in this way will it 
be possible to anive at a true and lasting solution of the problem. 

,' We must however be clear on this point — that the understanding 
of the phenomena of heredity is only possible on the fundamental 

(_ supposition of the continuity of the germ-plasm. The value of 



ON HEREDITY. 105 

experiment in relation to this question is somewhat doubtfuL 
A careful collection and arrangement of facts is far more likely 
to decide whether, and to what extent, the continuity of germ- 
plasm is reconcilable with the assumption of the transmission of 
acquired characters from the parent body to the germ, and from 
the germ to the body of the offspring. At present such trans- 
mission is neither proved as a fact, nor has its assumption been 
shown to be unquestionably necessary. 



III. 
LIFE AND DEATH. 

1883. 



LIFE AND DEATH. 



PREFACE. 

The following paper was first printed as an academic lecture in 
the summer of the present year (1883), with the title 'Upon the 
Eternal Duration of Life' ('tJber die Ewigkeit des Lebens'). In 
now bringing it before a larger public in an expanded and improved 
form, I have chosen a title which seemed to me to correspond 
better with the present contents of the paper. 

The stimulus which led to this biological investigation was 
given in a memoir by Gotte, in which this author opposes views 
which I had previously expressed. Although such an origin has 
naturally caused my paper to take the form of a reply, my inten- 
tion was not merely to controvert the views of my opponent, but 
rather — using those opposed views as a starting-point — to throw 
new light upon certain questions which demand consideration ; to 
give additional support to thoughts which I have previously ex- 
pressed, and to penetrate, if possible, more deeply into the problem 
of life and death. 

If, in making this attempt, the views of my opponent have been 
severely criticized, it will be acknowledged that the criticisms do 
not form the purpose of my paper, but only the means by which 
the way to a more correct understanding of the problems before us 
may be indicated. 



A. W. 



Freibueg I. Bkeisgau, 
Oct. 18, 1883. 



III. 

LIFE AND DEATH. 

In the, previous essay, entitled ' The Duration of Life,' I have 
endeavoured to show that the limitation of life in single individuals 
by death is not, as has been hitherto assumed, an inevitable phe- 
nomenon, essential to the very nature of life itself; but that it is 
an adaptation which first appeared when, in consequence of a certain 
complexity of structure, an unending life became disadvantageous to 
the species. I pointed out that we could not speak of natural 
death among unicellular animals, for their growth has no termina- 
tion which is comparable with death. The origin of new indivi- 
duals is not connected with the death of the old ; but increase by 
division takes place in such a way that the two parts into which 
an organism separates are exactly equivalent one to another, and 
neither of them is older or younger than the other. In this way 
countless numbers of individuals arise, each of which is as old as 
the species itself, while each possesses the capability of living on 
indefinitely, by means of division, 

I suggested that the Metazoa have lost this power of unending 
life by being constructed of numerous cells, and by the consequent 
division of labour which became established between the various 
cells of the body. Here also reproduction takes place by means 
of ceU-division, but every cell does not possess the power of 
reproducing the whole organism. The cells of the organism are 
differentiated into two essentially different groups, the reproductive 
cells — ova or spermatozoa, and the somatic cells, or cells of the 
body, in the narrower sense. The immortality of the unicellular 
organism has only passed over to the former ; the others must die, 
and since the body of the individual is chiefly composed of them, 
it must die also. 

I have endeavoured to explain this fact as an adaptation to the 
general conditions of life. In my opinion life became limited in 



112 LIFE AND DEATH. 

its duration, not because it was contrary to its very nature to be 
unlimited, but because an unlimited persistence of the individual 
would be a luxury without a purpose. Among unicellular organisms 
natural death was impossible, because the reproductive ceU and 
the individual were one and the same : among midticellular animals 
it was possible, and we see that it has arisen. 

Natural death appeared to me to be explicable on the principle of 
utility, as an adaptation. 

These opinions, to which I shaU return in greater detail in a 
later part of this paper, have been opposed by Gotte ^, who does not 
attribute death to utility, but considers it to be a necessity in- 
herent in life itself. He considers that it occurs not only in the 
Metazoa or multicellular animals, but also in unicellular forms of 
life, where it is represented by the process of encystment, which is 
to be regarded as the death of the individual. This encystment is a 
process of rejuvenescence, which, after a longer or shorter interval, 
interrupts multiplication by means of fission. According to Gotte, 
this process of rejuvenescence consists in the dissolution of the 
specific structure of the individual, or in the retrogression of the 
individual to a form of organic matter which is no longer living 
but which is comparable to the yolk of an egg. This matter is, by 
means of its internal energy, and in consequence of the law of 
growth which is inherent in its constitution, enabled to give rise to 
a new individual of the same species. Furthermore, the process of 
rejuvenescence among unicellular beings corresponds to the forma- 
tion of germs in the higher organisms. The phenomena of death 
were transmitted by heredity from the unicellular forms to the 
Metazoa when they arose. Death does not therefore appear for 
the first time in the Metazoa, but it is an extremely ancient 
process which ' goes back to the first origin of organic beings ' 
(l.c.,p. 8i). 

It is obvious, from this short resume, that Gotte's view is totally 
opposed to mine. Inasmuch as only one of these views can be 
fundamentally right, it is worth while to compare the two ; and 
although we cannot at present hope to explain the ultimate physio- 
logical processes which involve life and death, I think nevertheless 
that it is quite possible to aiTive at definite conclusions as to the 
general causes of these phenomena. At any rate, existing facts 
* ' Ueber den Ursprung ties Todes,' Hamburg and Leipzig, 1883. 



LIFE AND DEATH. 113 

have not been so completely thought out that it is useless to con- 
sider them once more. 

The question — what do we understand by death ? must be de- 
cided before we can speak of the origin of death . Gotte says, ' we 
are not able to explain this general expression quite definitely and 
in all its details, because the moment of death, or perhaps more 
exactly the moment when death is complete, can in no case be pre- 
cisely indicated. We can only say that in the death of the higher 
animals, all those phenomena which make up the life of the indivi- 
dual cease, and further that all the cells and elements of tissue which 
form the dead organism, die, and are resolved into their elements.' 

This definition would sufiice if it did not include that which is 
to be defined. For it assumes that under the expression ' dead 
organism ' we must include those organisms which have brought to 
an end the whole of their vital functions, but of which the component 
cells and elements may still be living. This view is afterwards 
more accurately explained, and in fact there is no doubt that the 
cessation of the activity of life in the multicellular organism rarely 
implies any direct connection with the cessation of vital functions in 
all its constituents. The question however arises, whether it is right 
or useful to limit the conception of death to the cessation of the 
functions of the organism. Our conceptions of death have been 
derived from the higher organisms alone, and hence it is quite 
possible that the conception may be too limited. The limitation 
might perhaps be removed by accurate and scientific comparison 
with the somewhat corresponding phenomena among unicellular 
organisms, and we might then arrive at a more comprehensive 
definition. Science has without doubt the right to make use of 
popular terms and conceptions, and by a more profound insight to 
widen or restrict them. But the main idea must always be retained, 
so that nothing quite new or strange may appear in the widened 
conception. The conception of death, as it has been expressed with 
perfect uniformity in all languages, has arisen from observations on 
the higher animals alone ; and it signifies not only the cessation of 
the vital functions of the whole organism, but at the same time 
the cessation of life in its single parts, as is shown by the impossi- 
bility of revival. The post-mortem death of the cells is also part 
of death, and was so, long before science established the fact that 
an organism is built up of numerous very minute living elements, 

I 



114 LIFE AND DEATH. 

of which the vital processes partially continue for some time after 
the cessation of those of the whole organism. It is precisely this 
incapacity on the part of the organism to reproduce the phenomena 
of life anew, which distinguishes genuine death from the arrest of 
life or trance ; and the incapacity depends upon the fact that the 
death of the cells and tissues follows upon the cessation of the 
vital functions as a whole. I would, for this reason, define death 
as an arrest of life, from which no lengthened revival, either of the 
whole or any of its parts, can take place ; or, to put it concisely, 
as a definite arrest of life. I believe that in this definition I have 
expressed the exact meaning of the conception which language has 
sought to convey in the word death. For our present purpose, the 
cause which gives rise to this phenomenon is of no importance, — 
whether it is simultaneous or successive in the various parts of the 
organism, whether it makes its appearance slowly or rapidly. For 
the conception itself it is also quite immaterial whether we are 
able to decide if death has really taken place in any particular 
case ; however uncertain we might be, the state which we call 
death would be not less sharply and definitely limited. We might 
consider the caterpillar of Euprepia flavia to be dead when frozen 
in ice, but if it recovered after thawing and became an imago, we 
should say that it had only been apparently dead, that life stood 
still for a time, but had not ceased for ever. It is only the irre- 
trievable loss of life in an organism which we call deathj and we 
ought to hold fast to this conception, so that it will not slip from 
ns, and become worthless, because we no longer know what we 
mean by it. 

We cannot escape this danger if we look upon the post-mortem 
death of the cells of the body as a phenomenon which may 
accompany death, but which may sometimes be wanting. An 
experiment might be made in which some part of a dead animal, 
such as the comb of a cock, might be transplanted, before the 
death of the cells, to some other living animal : such a part might 
live in its new position, thus showing that single members may 
survive after the appearance of death, as I understand it. But 
the objection might be raised that in such a case the cock's 
comb has become a member of another organism, so that it would 
be lost labour to insert a clause in our definition of death which 
would include this phenomenon. The same objection might be 



LIFE AND DEATH. 115 

raised if the transplantation took place a day or even a year before 
the death of the cock. 

Gotte is decidedly in error when he considers that the idea of 
death merely expresses an 'arrest of the sum of vital actions in 
the individual/ without at the same time including that definite 
arrest which involves the impossibility of any revival. De- 
composition is not quite essential to our definition, inasmuch as 
death may be followed by drying-up^, or by perpetual entombment 
in Siberian ice (as in the well-known case of the mammoth), or by 
digestion in the stomach of a beast of prey. But the notion of a 
dead body is indeed inseparably connected with that of death, 
and I believe that I was right in distinguishing between the division 
of an Infusorian into two daughter-cells, and the death of a Metazoon, 
which leaves offspring behind it, by calling attention to the absence 
of a dead body in the process of fission among Infusoria^. The 
real proof of death is that the organized substance which previously 
gave rise to the phenomena of life, for ever ceases to originate 
such phenomena. This, and this alone, is what mankind has 
hitherto understood by death, and we must start from this definition 
if we wish to retain a firm basis for our considerations. 

We must now consider whether this definition, derived from 
observation of higher animals, may be also applied without altera- 
tion to the lower, or whether the corresponding phenomena which 
arise in these latter, differ in detail from those of the higher 
animals, so that a narrower limitation of the above definition is 
rendered necessary. 

Gotte believes the process of encystment which takes place in 
so many unicellular animals (Monoplastides) to be the analogue of 
death. According to this authority, the individuals in question, 
not only undergo a kind of winter sleep — a period of latent life — 
but when surrounded by the cyst they lose their former specific 
organization ; they become a ' homogeneous substance,' and are 
resolved into a germ, from which, by a process of development, 
a new individual of the same species once more arises. The 
division of the contents of the cyst, viz. its multiplication, is, 
according to this view, of secondary importance, and the essential 

* As in the case of the bodies of monks on the Great St. Bernard, or the dried-up 
bodies in the well-known Capuchine Monastery at Palermo. 
' See below. 

I % 



116 LIFE AND DEATH. 

feature in the process is the rejuvenescence of the individual. This 
rejuvenescence however is said to not only consist in the simple 
transformation of the old individual, but in its death, followed 
by the building up anew of another individual. * The parent 
organism and its offspring are two successive living stages of the 
same substance — separated, and at the same time connected, by the 
condition of rejuvenescence which lies between them ' (1. c, p. 79). 
An ' absolute continuity of life does not exist ' ; it is only the dead 
organic matter which establishes the connection, and the * identity 
of this matter ensures heredity.' 

It is certainly surprising that Gotte should identify encystment 
with a cessation of life, and we may well inquire for the evidence 
which is believed to support such a view. The only evidence lies 
in a certain degree of degeneration in the structure of the individual, 
and in the cessation of the visible external phenomena of life, such 
as feeding and moving. Does Gotte really believe that it is an 
incorrect interpretation of the facts to assume that a vita minima 
continues to exist in the protoplasm, after its complexity has 
diminished ? Are we compelled to invoke a mystical explanation 
of the facts, by an appeal to such an indefinite principle as Gotte's 
rejuvenescence? Would not the oxygen, dissolved in the water, 
affect the organic substance the life of which it formerly maintained, 
and would it not cause its decomposition, if it were in reality dead ? 

I, too, hold that the division of the encysted mass is of 
secondary importance, and that the encystment itself, without the 
resulting multiplication, is the original and essential part of the 
phenomenon. But it does not follow from this that the encyst- 
ment should be considered as a process of rejuvenescence. What 
is there to be rejuvenated? Certainly not the substance of the 
animal, for nothing is added to it, and it can therefore acquire no 
new energy ; and the forms of energy which it manifests cannot be 
changed, since the form of the matter is just the same after quitting 
the cyst as it was before. Rejuvenescence has also been mentioned 
in connection with the process of conjugation, but this is quite 
another thing. It is quite reasonable, at least in a certain sense, to 
maintain the connection of rejuvenescence VAdth conjugation ; for 
a fusion of the substance of two individuals takes place, to a 
greater or lesser extent, in conjugation, and the matter which 
composes each individual is therefore really altered. But in simple 



LIFE AND DEATH. IIT" 

encystment, rejuvenescence can only be understood in the sense in 
which we speak of the fable of the Phoenix, which, when old, was 
believed to be consumed by fire, and to rise again from its own 
ashes as a young- bird. I doubt whether this idea is in agreement 
with the physiology of to-day, or with the laws of the conservation 
of energy. It is easy to pull down an old house with rotten beams 
and crumbling walls, but it would be impossible to build it anew 
with the old material, even if we used new mortar, represented in 
Gotte's hypothesis by water and oxygen. For these reasons I con- 
sider the idea of rejuvenescence of the encysted individual to be 
contrary to pur present physiological knowledge. 

It is much more simple and natural to regard encystment as 
adapted for the protection of certain individuals in a colony from 
destruction by being dried up or frozen, or for the protection of the 
individual dui'ing multiplication by division, when it is helpless, 
and would easily fall a prey to enemies, or to secure advantages in 
some other way^. The case of Actinosphaerium, mentioned by 
Gotte, clearly demonstrates that rejuvenescence of the individual is 
not the only event which happens during encystment, for this 
would scarcely require six months. The long duration of latent 
life, from summer to the next spring, clearly proves that encystment 
is of the highest importance for the species, in order to maintain the 
life of the individual through the dangers of an unfavourable season ^, 

^ Professor Gruber informs me that among the Infusoria of the harbour of 
Genoa, he has observed a species which encysts upon one of the free-swimming 
Copepoda. He has often found as many as ten cysts upon one of these Copepods, 
and has observed the escape of their contents whenever the water under the cover- 
glass began to putrefy. Here advantage is probably gained in the rapid transport of 
the cyst by the Crustacean. 

^ The views of most biologists who have worked at this subject agree in aU 
essentials with that expressed above. Biitschli says (Bronn's ' Klassen und Ordnungen 
des Thierrejchs,' Protozoa, p. 148) : 'The process of encystment does not appear to 
have originally borne any direct relation to reproduction : it appears on the contrary 
to have taken place originally, — as it frequently does at the present day, — either for 
the protection of the organism against injurious external influences, such as desicca- 
tion or the fatal eifects of impure water, etc. ; and also to enable the organism, 
after taking up an unusually abundant supply of food, to assimilate it in safety.' 
Balbiani ('Journ. de Micrographie,' Tom. V. x88i, p. 293) says in reference to the 
Infusoria, ' Un petit nombre d'espfeces, au lieu de se multiplier k I'etat de vie active, 
se reproduisent dans une sorte d'etat de repos, dit etat d'enkystement. Ces sortes 
de kystes peuvent etre designes sous le nom de kystes de reproduction, par opposition 
avec d'autres kystes, dans lesquels les Infusoires se renferment pour se soustraire k 
des conditions devenues defavorables du milieu qu'ils habitent, le manque d'air, le 
dessfechement, etc. — ceux-ci sont des kystes de conservation . . .' 



118 LIFE AND DEATH. 

When in this case, the specific organization degenerates to 
a certain extent, such changes depend in part upon the endeavour 
to diminish as far as possible the size of the organism — the pseu- 
dopodia being drawn in, while the vacuoles contract and com- 
pletely disappear. The degeneration may also, perhaps, depend in 
part upon the secretion of the cyst itself, which implies a certain 
loss of substance^. But degeneration chiefly depends upon the 
fact that the encystment is accompanied by reproduction in the 
way of fission, which seems to begin with a simplification of the 
organization, that is, with a fusion of the numerous nuclei. It is 
well known that many unicellular animals contain several nuclei — 
in other words, that the nuclear substance is scattered in small 
parts throughout the whole cell. But when the animal prepares 
for division, these pieces of nuclear substance fuse into a single 
nucleus which itself undergoes division into two equal parts ^ during 
the division of the animal. It is evident that the equal division 
of the whole nuclear substance only becomes possible in this way. 

There are, however, numerous cases which prove that the bodies 
of encysted animals may retain, during the whole process, exactly 
the same structure and differentiation, which were previously 
characteristic of them. Thus the large Infusorian T'dlina magna, de- 
scribed by Gruber, can be seen through the thin-walled cyst to 
retain the characteristic structure of its ectoplasm, and the whole of 
its organization. Even the movements of the enclosed animal 
do not cease ; it continues to rotate actively in the narrow cyst, 
as do the two or four parts into which it subsequently divides. 
Such observations prove that Gotte's view that ' every characteris- 
tic of the previous organization is lost,' is quite out of the question^ 
(1. c, p. 62). 

* This is of importance in so far as single individuals might be thus compelled to 
encyst even when the existing external conditions of life do not require it. The 
substance which Actinosphaerium, for example, employs in the secretion of its thick 
siliceous cyst must have been gradually accumulated by means of a process peculiar to 
the species. We can scarcely be in error if we assume that the silica accumulated 
in the organism cannot increase to an unlimited extent without injury to the other 
vital processes and that the secretion of the cyst must take place as soon as the 
accumulation has exceeded a certain limit. Thus we can understand that encyst- 
ment may occur without any external necessity. Similarly, certain Entomostraca 
(e.^. Moina) produce winter-eggs in a particular generation, and these are formed 
even when the animals are kept in a room protected from cold and desiccation. 

' Upon this point Professor Gruber intends to publish an elaborate memoir. 

^ This view has not even been proved for Actinosphaerium, upon which Gotte 



LIFE AND DEATH. 119 

For this reason I must strongly oppose Gotte's view that 
an encysted individual is a germ, viz. an organic mass still un- 
organized which can only become an adult individual by means of 
a process of development. I believe that an encysted individual is 
one possessing a protective membrane, in structure more or less 
simplified as an adaptation to the narrow space within the cyst, and 
to a possible subsequent increase by division, in short one in which 
active life is reduced to a minimum, and sometimes even completely 
in abeyance, as happens when it is frozen. 

j|[t is evident from the above considerations that encystment in 
no way corresponds with that which every one, including myself, 
understands by death, because during encystment one and the same 
being is first apparently dead and then again alive ; and we merely 
witness a condition of rest, from which active life will again 
emerge. This would remain true even if it were proved that life is, 
in reality, suspended for a time. But such proof is still wanting, 
and Gotte was apparently only influenced by theoretical considera- 
tions, when he imagmed that death intervened where unprejudiced 
observers have only recognised a condition of rest. He apparently 
entirely overlooked the fact that it is possible to test his views ; for 
all unicellular beings are in reality capable of dying : we can 
kill them, for example, by boiling, and they are then really dead and 
cannot be revived. But this state of the organism differs chemically 
and physically from the encysted condition, although we do not 
know all the details of the difference. The encysted animal, when 
placed in fresh water, presently originates a living individual, but 
the one killed by boiling only results in decomposition of the dead 
organic matter. Hence we see that the same external conditions 
give rise to different results in two different states of the organism. 
It cannot be right to apply the same term to two totally different 
states. There is only one phenomenon which can be called death, 
although it may be produced by widely diflferent causes. But if 
the encysted condition is not identical with the death which we 
can produce at will, then natural death, viz. that arising from 
internal causes, does not exist at all among unicellular organisms. 

These facts refute Gotte's peculiar view, which depends on the 

chiefly relies. The observations which we now possess merely indicate that the 
animal contracts to the smallest volume possible. Compare F. E. Schulze, ' Ehizo- 
podenstudien,' I, Arch. f. mikr. Anat. Bd. lo, p. 328 ; and Karl Brandt, ' Ueber 
Actinosphaeriun? Eichhomii,' Inaug. Diss. ; Halle, 1877. 



120 LIFE AND DEATH. 

existence of natural death among' the Monoplastid organisms; 
upon proof of the contradictory, his whole theory collapses. But 
there is nevertheless a certain interest in following it further, for 
we shall thus reach many ideas worthy of consideration. 

First, the question arises as to how death could have been 
transmitted from the Monoplastides ^ to the Polyplastides, a process 
which must have taken place according to Gotte. I will for the 
present omit the fact that I cannot accept the supposition that the 
process of encystment represents death. We may then inquire 
whether death has taken the place of encystment among the 
Polyplastides, or, if this is not the case, whether any process com- 
parable to encystment exists among the Polj'plastides. 

Gotte believes that death is always connected with reproduction, 
and is a consequence of the latter in both Protozoa and Metazoa. 
Reproduction has, in his opinion, a directly ' fatal effect,' and the 
reproducing individual must die. Thus the may-fly and the 
butterfly die directly after laying their eggs, and the male bee dies 
immediately after pairing ; the Orthonectides expire after expelling 
their germ-cells, while Magosphaera resolves itself into germ-cells, 
and nothing persists except these elements. It is but a step from 
this latter organism to the unicellular animals which transform 
themselves as a whole into germ-cells ; but in order to achieve 
this they must undergo the process of rejuvenescence, which Gotte 
assumes to be the same as death. 

These views contain many fallacies quite apart from the sound- 
ness or unsoundness of their foundation. The process of encystment, 
as Gotte thinks, represents, in the Monoplastides, true reproduction 
to which multiplication by means of division has been secondarily 
added. This encystment cannot be dispensed with, for internal 
causes determine that it must occasionally interrupt the process of 
multiplication by simple division. But, on the other hand, Gotte 
also considers the division of the contents of the cyst to be a 
secondary process. The essential characteristic of encystment is a 
simple process of rejuvenescence without multiplication. Hence 
we are forced to accept a primitive . condition in which simple 
division as well as the division of the encysted individual were 

* The conception of Protozoa and Metazoa does not correspond exactly with that 
of unicellular and multicellular beings, for which Gotte has proposed the names 
Mono- and Polyplastides. 



LIFE AND DEATH. 121 

absent, and in which reproduction consisted only in an often- 
repeated process of rejuvenescence among existing individuals, 
without any increase in their number. Such a condition is in- 
conceivable because it would involve a rapid disappearance of the 
species, and the whole consideration clearly shows us that division 
of un-encysted individuals must have existed from the first, and 
that this, and not a vague and mysterious rejuvenescence, has 
always been the real and primitive reproduction of the Mono- 
plastides. The fact that encystment does not always lead to the 
division of the contents of the cyst proves, in my opinion, that not 
reproduction but preservation against injury from without, was the 
primitive meaning of encystment. It is possible that at the present 
time there are but few Monoplastides which are able to go through 
an infinite number of divisions without the interposition of the 
resting condition implied by encystment ; although it has not yet 
been demonstrated for all species^. But it is not right to conclude 
from this that there is an internal necessity which leads to encyst- 
ment, that is to say to identify this process with rejuvenescence. It 
is much more probable that encystment is merely an adaptation 
to continual changes in the external Conditions of life, such as 
drought and frost, and perhaps also the want of food which arises 
from the over-population of small areas. The same phenomenon is 
known in certain low Crustacea — the DapJinidae — which possess 
an ephippium or protective case for their winter-eggs. This case is 
only developed after a certain definite number of generations has 
been run through, an event which may happen at any time in 
the year in species living in pools which are liable to be often 
dried-up ; but only in the autumn in such as live in lakes which 
are never dry. No one ever doubted that the periodical formation 
of the ephippium in certain generations was an adaptation to 
changes in the external conditions of life. 

Even if the process of rejuvenescence in the Monoplastides were 
really equivalent to the death of the higher animals, we could 
not conclude from this that it is necessarily associated with re- 
production. Encystment alone is not reproduction, and it first 

' Among the Rhizopoda encystment is only known in fresh-water forms, and 
not in a single one of the far more numerous marine forms which possess shells (see 
Biitschli, ' Protozoa,' p. 148) ; the marine Rhizopoda are not exposed to the effects 
of desiccation or frost, and thus the strongest motives for the process of encystment 
do not exist, at least among forms possessing a shell. 



122 LIFE AND DEATH. 

becomes a form of reproduction when it is associated with the 
division of the encysted animal. Simple division was the true 
and original form of reproduction in Monoplastides, and even now 
it is the principal and fundamental form. 

Hence we see that among the Monoplastides reproduction is not 
connected with death, even if we accept Gotte's view and allow 
that encystment represents death. I shall return later on to the 
relation between death and reproduction in the Metazoa ; but the 
question first arises whether encystment, if it is not death, has any 
analogue in the higher animals, and further whether death takes 
that place in their development which is occupied by encystment in 
the Monoplastides. 

Among the higher Metazoa there can be no doubt as to what 
we mean by death, but the precise nature of that which dies is not 
equally evident, and the popular conception is not sufficient for us. 
It is necessary to distinguish between the mortal and the im- 
mortal part of the individual — the body in its narrower sense 
(soma) and the germ-cells. Death only affects the former ; the 
germ-cells are potentially immortal, in so far as they are able, 
under favourable circumstances, to develope into a new individual, 
or, in other words, to surround themselves with a new body 
(soma) ^. 

But how is it with the lowest Polyplastides in which there is no 
antithesis between the somatic and germ-cells, and among which 
each of the component cells of the multicellular body has retained 
all the animal functions of the Monoplastides, even including re- 
production ? 

Gotte believes that the natural death of these organisms (which 
he rightly calls Homoplastides) consists in ' the dissolution of 
the cell-colony.' As an example of such dissolution Gotte takes 
Hackel's Magospliaera p^anula, a marine free-swimming organism 
in the form of a sphere composed of a single layer of ciliated cells, 

* I trust that it will not be objected that the genn-cells cannot be immortal, be- 
cause they frequently perish in large numbers, as a result of the natural death of the 
individual. There are certain definite conditions under which alone a germ-cell can 
render its potential immortality actual, and these conditions are for the most part 
fulfilled with difficulty (fertilization, etc.). It follows from tliis fact that the germ- 
cells must always be produced in numbers which reach some very high multiple of 
the necessary number of offspring, if these latter are to be ensured for the species. 
If in the natural death of the individual the germ-cells must also die, the natura 
death of the soma, becomes a cause of accidental death to the germ-cells. 



LIFE AND DEATH. 



128 



imbedded in a jelly. (For figure see below.) This organism 
cannot however be ' considered as a genuine perfect Polyplastid, 
for at a certain time the component cells part from one another 
and then continue to live independently in the condition of Mono- 
plastides.' These free amoebiform organisms increase considerably 
in size, encyst, and finally undergo numerous divisions — a kind of 
segmentation within the cyst. The result of the division is a 
sphere of ciliated cells similar to that with which the cycle began. 
In fact, Magosphaera is not a perfect Polyplastid, but a transitional 




Development of Magosfhaera planula (after Hackel). 

I. Encysted amoeboid form. 2 and 3. Two stages in the division of the same, 
4. Free ciliated sphere, the cells of which are connected by a gelatinous mass. 5. 
One of the ciliated cells which has become free by the breaking up of the sphere. 
6. The same in the amoeboid form. 7. The same grown to a larger size. 

form between Polyplastides and Monoplastides, as the discoverer of 
the group of animals of which it is the only representative, indi- 
cated, when he named the group ' Catallacta.' 

According to Gotte, the natural death of Magosfhaera consists, 
as in the undoubted Protozoa, in a process of rejuvenescence by 
encystment. The dissolution of the ciliated sphere into single cells 
'cannot be identical with natural death. For the regular and 



124 LIFE AND DEATH. 

complete separation of the Magosphaera-ceWs proves that their in- 
dividuality has not been completely subordinated to that of the 
whole colony, and it proves that the latter is not completely 
individualised ^.* 

Nothing- can be said against this, if we agree in identifying 
death with the encystment of the Monoplastides. Now we could, 
as Gotte rightly remarks, derive the lower forms of Polyplastides 
from Magosphaera if ' the connection between the cells of the 
ciliated sphere were retained until encystment, viz. until the re- 
production of the single cells had taken place ^.' After this had 
been accomplished, Gotte considers that death would consist ' in the 
complete separation of the cells from one another, accompanied in 
all probability by their simultaneous change into germ-cells.' 
The fallacy in this is evident ; if death is represented in one 
case by the encystment during which single cells change into 
germ-cells, then this must apply to the other case also, for nothing 
has changed except the duration of the cell-colony. The nature 
of encystment cannot be affected by the fact that the cells separate 
from one another a little earlier or a little later. If it is true 
that death is represented by encystment among the Monoplastides, 
then the same conclusion must also hold for the Polyplastides ; or 
rather death must be represented in them by the process of re- 
juvenescence, which Gotte considers to be the essential part of 
encystment. Gotte ought not to identify death with the dissolu- 
tion of the cell-colony of which the lowest and highest Poly- 
plastides are alike composed ; but he should seek it in the process 
of rejuvenescence which takes place within the germ-cells. If it is 
essential to the nature of reproduction that the cells set apart for 
that purpose should pass through a process of rejuvenescence, which 
is equivalent to death, then this must be true for the reproductive 
cells of all organisms. If these conclusions hold good, there is 
nothing to prevent us from assuming that such a process of rejuve- 
nescence actually occurs in the higher animals. Gotte evidently 
holds this view, as is plainly shown in the last pages of his essay. 
He there attempts to bring his views of the death and rejuve- 
nescence of the germ into harmony with his previously developed 
idea of the derivation of death among the Polyplastides from the 
dissolution of the cell-colonies. Gotte still cling-s to the view 
' 1. c, p. 78. » 1. c, p. 47. 



LIFE AND DEATH. 125 

which he propounded in describing the development of Bomhinator, 
according to which the egg-cell of the higher Metazoa must pass 
through a process of rejuvenescence representing death, before it 
can become a germ. 

According to Gotte's ^ idea ' the egg of a Bomhinator igneits before 
fertilization cannot be considered to be a cell either wholly or in 
part ; and this is equally true of it at its origin and after its complete 
development ; it is only an essentially homogeneous organic mass 
enclosed by a membrane which has been deposited externally.' 
This mass is ' unorganised and not living ^/ and * during the first 
phenomena of its development all vital powers must be excluded.' 
In this way the continuity of life between two successive in- 
dividuals is always interrupted ; or, as Gotte says in his last 
essay : — ' The continuity of life between individuals of which one 
is derived from the other by means of reproduction, exists neither 
in the rejuvenescence of the Monoplastides nor in the condition 
of the germ among the Polyplastides — a condition which is derived 
from the former ^. 

This is quite logical, although in my opinion it is both un- 
proved and incorrect. But, on the other hand, it is certainly 
illogical for Gotte to derive the death of the Metazoa in a totally 
different way, i. e. from the dissolution of their cell-colonies. It is 
quite plain that the death of the Metazoa does not especially 
concern the reproductive cells, but the individual which bears them ; 
Gotte must therefore seek for some other origin of death — an 
origin which will enable it to reach the body (soma) — as opposed 
to the germ-cells. If there still remained any doubt about the 
failure to establish a correspondence between death and the encyst- 
ment of the Monoplastides, we have here, at any rate, a final demon- 
stration of the failure ! 

But there is yet another great fallacy concealed in this derivation 
of the death of the Polyplastides. 

Among the lowest Polyplastides, where all the cells still remain 
similar, and where each cell is also a reproductive cell, the dissolu- 
tion of the cell-colony is, according to Gotte, to be regarded as death, 
inasmuch as ' the integrity of the mother-individual absolutely 

* ' Entwicklungsgeschichte der Unke,' Leipzig, 1875, p. 65. 

^ Id,, p. 843. 

^ ' Uxsprung des Todes,' p. 79. 



126 LIFE AND DEATH. 

comes to an end' (1. c., p. 78). The dissolution of a cell-colony into 
its component living- elements can only be called death in the most 
figurative sense, and can have nothing to do with the real death of 
the individuals ; it only consists in a change from a higher to a 
lower stage of individuality. Could we not kill a Magosphaera 
by boiling or by some other artificial means, and would not the 
state which followed be death? Even if we define death as an 
arrest of life, the dissolution of Magosphaera into many single cells 
which still live, is not death, for life does not cease in the organic 
matter of which the sphere was composed, but expresses itself in 
another form. It is mere sophistry to say that life ceases because 
the cells are no longer combined into a colony. Life does not in 
truth cease for a moment. Nothing concrete dies in the dissolution 
of Magosphaera ; there is no death of a cell-colony, but only of a 
conception. The Homoplastides, that is cell-colonies built up of 
equal cells, have not yet gained any natural death, because each of 
their cells is, at the same time, a somatic as well as a reproductive 
cell : and they cannot be subject to natural death, or the species 
would become extinct. 

It is more to the purpose that Gotte has sought for an illus- 
tration of death among those remarkable parasites, the Ortho- 
nectides, because in them we do at any rate meet with real 
death. They are indeed very low organisms ; but nevertheless 
they stand far above Magosphaera^ even if the latter were hypo- 
thetically perfected up to the level of a true Homoplastid, for the 
cells which compose the body of the Orthonectides are not all similar, 
but are so far differentiated that they are even arranged in the 
primitive germ-layers, and a form results which has rightly been 
compared with that of the Gastrula. It is true they are not quite 
so simple as Gotte ^ figures them, for they not only consist of ecto- 
derm and germ-cells, but, according to Julin^, the endoderm is 
arranged in two layers — the germ-cells and a layer which forms 
during development a strong muscular coat ; and in the second 
female form the egg-cells are surrounded by a tolerably thick 
granular tissue. There is nevertheless no doubt that in the first 
female form, when sexually mature, the greater part, not only of the 

' 1. c, p. 42. 

' ' Contributions k iTiistoire des Mesozoaire?. Recherches sur I'organisation et 
le d^veloppement embryonnaire des Orthonectides/ Arch, de Biologie, vol. iii. 1882. 



LIFE AND DEATH. 



127 



endoderm but of the whole body, is made up of ova, so that the 
animal resembles a thin-walled sac fall of eggs. The ova escape 
by the bursting of the thin ectoderm, and when they have all 




Obthonectides (after Jnlin). 

8. First female form : the cap-like anterior part has become detached and the egg- 
cells {eiz) are escaping. 9. Second female form: et2 = egg- cells; outside these are 
the muscular layer {m) and the ectoderm (eld). 10 and 1 1. Two fragments of such a 
female broken to pieces by spontaneous division : the egg-cells are embedded in 
a granular mass, and undergo embryonic development in it at a later period ; the 
whole is surrounded by ciliated cells. 12. Male discharging the spermatozoa by the 
breaking up of the actoderm (eM) ; sp spermatozoa ; m muscle. 



128 LIFE AND DEATH. 

escaped, the thili disintegrated membrane, composed of ciliated 
cells, is no long-er in a condition to live, and dies at once. This is 
the course of events as described by Gotte, and he is probably 
correct in his interpretation. This is the real death of the Ortho- 
nectides, and if we regard them as low primitive forms (Mesozoa), 
here for the first time in the ascending series we meet with natural 
death. But the causes of this are scarcely so clear as Gotte seems 
to think when he ascribes it to the effect of reproduction — an 
effect which is ' not only empirically necessary, but absolutely 
unavoidable.' Such a necessity is explained by the fact that the 
endoderm consists entirely of germ-cells. Now the life of the 
organism, being dependent upon the mutual action of both layers, 
must cease as soon as the whole endoderm is extruded during repro- 
duction. 

Arguments such as these pass over the presence of a mesoderm ; 
but apart from this omission, it does not appear to me so self- 
evident from a purely physiological standpoint, that the ectodermal 
sheath with its muscle layer must die after the extrusion of the 
germ-cells. 

In those females to which Gotte refers in this passage, the whole 
sheath remains at first quite uninjured, with the exception of a 
small cap at the anterior end, which is pushed off to give exit to 
the ova ; and inasmuch as the sheath continues to swim about in 
the nutritive fluids after this has taken place, the proof is at any 
rate wanting that it cannot support itself quite as well as before, 
although it has lost the germ-cells. 

Then why does it die ? Mv answer to this is simple ; — ^because 
it has lived its time ; because its length of life is limited to a 
period which corresponds with the time necessary for complete 
reproduction. The physical constitution of the body is so regulated 
that it remains capable of living until the extrusion of the repro- 
ductive cells, and then dies, however favourable external conditions 
may be for its further support. 

The correctness of this explanation is shown by a consideration 
of the males and the second form of females ; for in these cases the 
body falls to pieces, not as a consequence of reproduction, but as a 
preparation for it ! 

Gotte only mentions the second female form in a note, in which 
he says, it appears ' that in the second female form of these animals 



LIFE AND DEATH. 129 

the whole body breaks into many pieces, and the superficial layer 
gradually atrophies, so that it dies before the eggs are extruded.' 
In Julin's account ^, upon which Gotte bases his statements, there 
are, however, some not unimportant differences. For instance, the 
eggs are not extruded at all, but embryonic development takes 
place within the body of the mother, which has previously under- 
gone spontaneous division into several pieces. In this case, the eggs 
diflPer from those of the other female form, inasmuch as they do not 
constitute the whole of the endoderm, but are embedded (as was 
stated above) in a fairly voluminous granular mass at the expense 
of which, or at least by means of which, they are nourished ; for 
they increase considerably in size during their development. But 
not only this granular mass, but ?11 the layers of the body of the 
mother, even the ectoderm, persist during the embryonic develop- 
ment of the offspring. Indeed, the ectoderm must continue to 
grow during the division of the mother animal, for it gradually 
covers in the products of division on all sides, and, by means of 
its cilia, causes the animal to swim about in the fluids of its host. 
After some time the cilia are lost, and the separate parts into 
which the mother animal has divided, fix themselves upon some part 
of the body-cavity of the host ; the young become free, and the re- 
mains of the body of the mother probably disappear by dissolution 
and resorption ^. In this case the remains of the mother animal seem 
to be, to some extent, consumed by the embryos, — a process which 
sometimes, although very rarely, happens elsewhere. We can 
scarcely consider this as a primitive arrangement, or look upon 
it as a proof that ' reproduction ' has a necessarily fatal effect upon 
the Polyplastid organism. 

In the male, the mass of spermatozoa does not swell out the 
body to such an extent that its walls must give way and thus 
permit an exit, but the large ectoderm cells atrophy spontaneously 
at the time of maturity, and as they fall off, exit is given to the 
spermatozoa here and there. In this instance also the dissolution 
of the body is not a consequence of reproduction, but reproduction can 
only take place when the dissolution of the body has preceded it ! 
' 1. c, p. 37. 

^ Julin does not enter into further details on this point, and it is not quite clear at 
what precise time the cells of the ectoderm atrophy ; but this is irrelevant to the 
origin of death, since the granular mass surrounding the egg-cells at any rate belongs 
to the soma of the mother. 

K 



130 LIFE AND DEATH. 

In this remarkable arrangement we cannot discern anything" 
except an evident adaptation of the life of the body-cells to repro- 
ductive purposes, and this adaptation was rendered possible because, 
after the evacuation of the sexual cells, the body ceased to be of any 
value for the maintenance of the species. 

But even if we assume, that the death of the Orthonectides is, 
in Gotte's sense, a consequence of reproduction, inasmuch as, in the 
two forms of females as well as in the male, the extrusion of a mass 
of developed germ-cells or embryos deprives the organism of the 
physiological possibility of living longer, how can we explain the 
necessity of death as a direct consequence of reproduction in 
all Polyplastides ? Is the body — the soma — of the Metazoa so im- 
perfectly developed, as compared with the reproductive cells, that 
the extrusion of the latter involves the death of the former ? As a 
matter of fact in the majority of cases the relations are reversed ; 
the number of body-cells usually exceeds the germ-cells a hundred- 
or a thousand-fold, and the body is, as regards nutrition, so com- 
pletely independent of the reproductive cells, that it need not be 
in the least disadvantageously affected by their extrusion. And 
if the Orthonectid-like ancestors of the Metazoa were compelled 
to give up their insignificant somatic part after the extrusion 
of their germ-cells, because it could now no longer support itself, 
does it therefore follow that the somatic cells had for ever lost the 
power of surviving, even when their phyletic descendants were sur- 
rounded by more favourable conditions ? Had they to inherit ' the 
necessity of death ' for all time ? Whence came this great change in 
the nature of organisms which, before the differentiation of Homo- 
plastids into Heteroplastids, were endowed with the immortality of 
unicellular beings ? 

And it must be remembered that it is only an assumption which 
places the Orthonectides among the lowest Metazoa (Heteroplastids). 
I do not intend to greatly emphasize this point, but the formation 
of the Gastrula by embole, and the absence of a mouth and ali- 
mentary canal, shows that these parasites are extremely degenerate, 
and the same may be said of almost all endoparasites. The Gas- 
trula, as an independent organism, was without doubt primitively 
provided with both mouth and stomach, and the mass of ova 
filling the female Orthonoctid is an adaptation to a parasitic life, 
which on the one side renders the possession of a stomach a super- 



LIFE AND DEATH. 131 

fluity, and on the other demands the production of a great number 
of germ-cells^. It is certain that the Orthonectides, as at present 
constituted, cannot have lived in the free condition, and also that 
their adaptation to parasitism cannot have arisen at the beginning 
of the phyletic development of Metazoa, because they inhabit star- 
fishes and Nemertines — both relatively highly developed Metazoa. 
Hence it is, at any rate, doubtful whether the Orthonectides can 
claim to pass as typical forms of the lowest Heteroplastids, and 
whether their reproduction can be looked upon -' as typical for the 
unknown ancestors of all Polyplastids ' (1. c, p. 45). If, however, we 
accept some organism resembling these Orthonectides as the most 
ancient Heteroplastid, being a free-living organism, it must have 
had a stomach, and the cells surrounding it must — as a whole or in 
part — have possessed the power of digesting ; at any rate, they 
cannot aU have been germ-cells, and therefore it is improbable that 
death would be the direct result of the extrusion of the germ-cells. 
Let us now consider the manner in which Gotte has endeavoured 
to explain the transmission of the cause of death — which first 
appeared in the Orthonectides — from these organisms to all later 
Metazoa, until the very highest forms are reached. Exact proofs 
of this supposition are unfortunately wanting, and the evidence is 
confined to the collection of a number of cases in which death and 
reproduction take place nearly or quite simultaneously. These 
would prove nothing, even if post Jioc were always projder Jioc ; and 
there are, opposed to them, a number of cases in which reproduction 
and death take place at different times. In obtaining evidence for 
' the fatal infiuenee of reproduction,' is it possible to point to every 
case of sudden death after the act of oviposition or fertilization ? 
These cases occur among many of the higher animals, especially in 
Insects, and were collected by me in an earlier work^. It is 

^ Leuckart finds such a great resemblance between the newly born young of 
Distoma and the Orthonectides, that he is inclined to believe that the latter are 
Trematodes, ' which in spite of sexual maturity have not developed further than the 
embryonic condition of the Distoma^ (' Zur Entwicklungsgeschichte des Leberegels,' 
Zool. Anzeiger, 1881, No. 99). In reference to the Dicyemidae, which resemble the 
Orthonectides in their manner of living and in their structure, Gegenbaur has stated 
his opinion that they belong to a ' stage in the development of Platyhelminthes ' 
(Grundriss d. vergleich. Anatomic). Giard includes both in the 'phylum Vermes,' 
and regards them as much degenerated by parasitism ; and Whitman — the latest inves- 
tigator of the Dicyemids — speaks of them in a similar manner in his excellent work 
* Contributions to the Life-history and Classification of Dicyemids' (Leipzig, 1882). 

* ' Dauer dea Lebens ; ' translated as the first essay in this volume. 

K 3 



132 LIFE AND DEATH. 

obvious that such cases are exceptional, but in a restricted sense it 
is quite true, as far as these individual instances are concerned, 
that death appears as a consequence of reproduction. The male bee, 
which invariably dies while pairing-, is undoubtedly killed in con- 
sequence of a very powerful nervous shock ; and the female Psychid, 
which has laid all her egg's at once, dies of ' exhaustion ' — however 
we may attempt to explain the term on physiological principles. 

Can we conclude from these cases that the effects of reproduction 
are, in Gotte's sense, universally fatal ; that reproduction is the 
positive and 'exclusive explanation of natural death'? (1. c, p. 32.) 
I need not linger over these isolated examples, but I turn at once 
to the foundation of the whole conclusion — a foundation which is 
obviously unable to support the superstructure erected on it. 
Gotte formally derives the idea that death is a necessary condition 
of reproduction, from a very heterogeneous collection of facts. 
When we examine this collection we find that the process which is 
taken to be death is not the same thing in all these instances, 
while the same is true of the influence of reproduction by which 
death is supposed to be caused. The whole conception arises out of 
the process of encystment, which is regarded as the building-up of 
reproductive material — that is, as true reproduction ; and since, ac- 
cording to Gotte's view, the formation of germs is always inti- 
mately connected with an arrest of life, and since, by his own 
definition, this stand-still of Kfe is equivalent to death, it follows 
that, with such a theory, reproduction, in its essential nature, must 
be inseparably connected with death. It is necessary at this juncture 
to remember what Gotte means by the process of rejuvenescence, 
and to point out that he is dealing with something quite different 
from ' the fatal influence of reproduction,' which was just now men- 
tioned with regard to insects. ' Rejuvenescence,' bound up as it is 
with encystment and reproduction, is, according to Gotte, 'a re- 
coining of the specific protoplasm, by means of which the identity 
of its substance is fixed by heredity,' a ' marvellous process in which 
phenomena the most important in the whole life of the animal, 
and in fact of all organisms — reproduction and death — have their 
roots' (1. c, p. 81). Whether such re-coining really takes place or 
not, at any rate I claim to have shown above that it does not cor- 
respond with death in the Metazoa, and — if it is represented at all 
in these latter — that it ought to be looked for in the reproductive 



LIFE AND DEATH. 133 

cells ; and indeed, in anotlier passage, Gotte himself has placed the 
process in these cells. 

While, among" the Monoplastids, according to Gotte, the causes 
of the supposed death lie hidden in this mysterious change of the 
organism into reproductive material, Gotte asserts that among the 
Polyplastids (such as Magosphaera, hypothetically perfected so as 
to form a genuine Polyplastid), the causes of death operate so 
that the organism breaks up into its component cells, all these 
being still reproductive cells — a process which, unlike ' rejuvenes- 
cence,' has nothing mysterious about it, and which is certainly not 
genuine death. In the Orthonectid-like animals death does not 
occur as a consequence of the dispersal of the reproductive cells, 
but rather because the part of the animal which remains is so 
small and effete that, being unable to support itself, it necessarily 
dies. From this point at least the object of death and the con- 
ception of it remain the same, but now the idea of reproduction 
undergoes a change. When the Rhabdite females of Ascaris are 
eaten up by their offspring, is this mode of death connected with 
the ' rejuvenescence of protoplasm ' ? (1. c, p. 34.) Is there any 
deep underlying relationship between such an end and the essential 
nature of reproduction? The same question may be asked with 
regard to the 'Redia or the Sporocyst of Trematodes, which are 
converted into slowly dying sacs during the growth of the Cer- 
cariae within them.' We cannot speak of the ' fatal influence 
of reproduction' among tape-worms just because 'in the ripe seg- 
ments the whole organization degenerates under the influence of 
the excessive growth of the uterus.' It certainly degenerates, but 
only so far as the developing mass of eggs demands. In fact, at 
a sufficiently high temperature, death does not occur, and such 
mature segments of tape-worms creep about of their own accord. 
We cannot fail to recognize that in this as well as in the above- 
mentioned cases we have to do with adaptation to certain very 
special conditions of existence — an adaptation leading to an im- 
mense development of reproductive cells in a mother organism which 
can no longer take in nourishment, or which has become entirely 
superfluous because its duty to its species is already fulfilled. If 
this is an example of death inherent in the essential nature of re- 
production, then so is the death of a mature segment of a tape- 
worm in the gastric juices of the pig that eats it. 



134 LIFE AND DEATH. 

With Gotte, the conception of reproduction, like the conception of 
death, is a protean form, which he welcomes in any shape, if only- 
he can use it as evidence. If death is a necessary consequence of 
reproduction, its cause must be always essentially the same, and 
might be expressed in one of the following suggestions: — (i) in 
the necessity for a 're-coining' of the protoplasm of the germ- 
cells ; but here death could only affect the germ-cells themselves : 
(a) perhaps in the withdrawal of nourishment by the mass of 
developing reproductive material, just as death occurs sometimes 
among men by the excessive drain on the system caused by morbid 
tumours : (3) or in consequence of the development of the off- 
spring in the body of the mother ; this however would only affect 
the females, and could therefore have no deep and general signifi- 
cance : (4) from the extrusion of the sexual cells, — ova or sper- 
matozoa, — and in the impossibility of further nourishment which is 
consequent upon this extrusion — (Orthonectides ?) : or (5) finally in 
an excessively powerful nervous shock brought about by the ejection 
of the reproductive cells. 

But no one of these alternatives is the universal and inevitable 
cause of death. This proves irrefutably that death does not proceed 
as ail intrinsic necessity from reproduction, although it may be 
connected with the latter, sometimes in one way and sometimes 
in another. But we must not overlook the fact that in many 
cases death is not connected with reproduction at all ; for many 
Metazoa survive for a longer or shorter period after the repro- 
ductive processes have ceased. 

In fact, I believe I have definitely shown that no process exists 
among unicellular animals which is at all comparable with the 
natural death of the higher organisms. Natural death first ap- 
peared among multicellular beings, and among these first in the 
Heteroplastids. Furthermore, it was not introduced from any 
absolute intrinsic necessity inherent in the nature of living matter, 
but on grounds of utility, that is from necessities which sprang 
lip, not from the general conditions of life, but from those special 
conditions which dominate the life of multicellular organisms. 
If this were not so, unicellular beings must also have been en- 
dowed with natural death. I have already expressed these ideas 
elsewhere^, and have briefly indicated how far, in my opinion, 
^ See the first essay upon ' The Duration of Life,' p. 22 et ge<i. 



LIFE AND DEATH. 135 

natural death is expedient for multicellular organisms. I found 
the essential reason for confining the life of the Metazoa to a 
fixed and limited period, in the wear and tear to which an indi- 
vidual is exposed in the course of a life -time. For this reason, 
'the longer the individual lived, the more defective and crippled 
it would become, and the less perfectly would it fulfil the purpose 
of its species ' (1. c, p. 24). Death seemed to me to be expedient 
since * worn-out individuals are not only valueless to the species, 
but they are even harmful, for they take the place of those which 
are sound ' (1. c, p. 24). 

I still adhere entirely to this explanation ; not of course in the 
sense that an actual physical struggle has ever taken place between 
the mortal and immortal varieties of any species. If Gotte under- 
stood me thus, he may be justified by the brief explanations given 
in the essay to which I have alluded ; but when he also attributes 
to me the opinion that such hypothetically immortal Metazoa had 
but a very limited period for reproduction, I fail to see what part 
of the essay in question can be brought forward in support of his 
statement. Only under some such supposition can I be reproached 
with having assumed the existence of a process of natural selection 
which could never be effective, because any advantage which accrued 
to the species from the shortening of the duration of life could not 
make itself felt in a more rapid propagation of the short-lived 
individuals. The statement ' that in this and in every other case 
it is a sufficient explanation of the processes of natural selection 
to render it probable that any kind of advantage is gained ' ^ is 
indeed erroneous. The explanation ought rather to be 'that the 
forms in question would for ever transmit their characters to a 
greater number of descendants than the other forms.' I have not 
however as yet attempted to think out in detail such processes of 
natural selection as would limit the somatic part of the Metazoan 
body to a short term of existence, and I only wished to emphasize 
the general principle lying at the basis of the whole process, with- 
out stating the precise manner in which it operates. 

If I now attempt to take this course, and to reconstruct theo- 
retically the gradual appearance of natural death in the Metazoa, 
I must begin by again alluding to Gdtte's criticisms in reference 
to the operation of natural selection. 

' ' Ursprung des Todes,' p. 29. 



136 LIFE AND DEATH. 

I consider death as an adaptation, and believe that it has arisen 
by the operation of natural selection. Gtitte ^, however, concludes 
from this that 'the first origin of hereditary and consequently 
(for the org-anization in question) necessary death, is not explained 
but already assumed.' ' The operation and sig-nificance of the 
principle of utility consists in selecting the fittest from among 
the structures and processes which are at hand, and not in directly 
creating new ones. Every new structure arises at first, quite 
independently of any utility, from certain material causes present 
in a number of individuals, and when it has proved useful and is 
transmitted, it extends, according to the laws of the survival of the 
fittest, in the group of animals in which it appeared. This exten- 
sion will undergo further increase with every advance in utility 
which results from further structural changes, until it extends 
over the whole group. So that usefulness effects the preservation 
and the distribution of new structures, but has nothing whatever 
to do with the causes of their primary origin and their consequent 
transmission to all other individuals. Indeed, on these hereditary 
causes the necessity of the structures in question depends, so that 
their usefulness in no way explains their necessity.' 

' These conclusions, when applied to the origin of natural death 
called forth by internal causes, would show that it became inevitable 
and hereditary in a number of the originally immortal Metazoa, 
before there could be any question as to the benefits derived from its 
influence. Such influence must have consisted in the fact that more 
descendants survived the struggle for existence and were able to 
enter upon reproduction among the individuals which had inherited 
the predisposition to die than among the potentially immortal 
beings which would be damaged in the struggle for existence, 
and would therefore be exposed to still further injuries. The exist- 
ing necessity for natural death in all Metazoa might therefore be 
derived in an unbroken line of descent from the first mortal 
Metozoan, of which the death became inevitable from internal 
causes, before the principle of utility could operate in favour of its 
dissemination.' 

In reply to this I would urge : that it has been very often 
maintained that natural selection can produce nothing new, but 
can only bring to the front something which existed previously to 

' 1. c, p. 5. 



LIFE AND DEATH. 137 

the exercise of choice ; but this argument is only true in a very- 
limited sense. The complex world of plants and animals which we 
see around us contains much that we should call new in comparison 
with the primitive beings from which, as we believe, everything 
has developed by means of natural selection. No leaves or flowers, 
no digestive system, no lungs, legs, wings, bones or muscles were 
present in the primitive forms, and all these must have arisen 
from them according to the principle of natural selection. These 
primitive forms were in a certain sense predestined to develope 
them, but only as possibilities, and not of necessity ; nor were they 
preformed in them. The course of development, as it actually took 
place, first became a necessity by the action of natural selection, 
that is by the choice of various possibilities, according to their 
usefulness in fitting the organism for its external conditions of life. 
If we once accept the principle of natural selection, then we must 
admit that it really can create new structures, instincts, etc., not 
suddenly or discontinuously, but working by the smallest stages 
upon the variations that appear. These changes or variations must 
be looked upon as very insignificant, and are, as I have of late 
attempted to show ^, quantitative in their nature ; and it is only 
by their accumulation that changes arise which are sufficiently 
striking to attract our attention, so that we call them ' new * 
organs, instincts, etc. 

These processes may be compared to a man on a journey who pro- 
ceeds from a certain point on foot by short stages, at any given time, 
and in any direction. Pie has then the choice of an infinite number 
of routes over the whole earth. If such a man begins his wanderings 
in obedience to the impulse of his own will, his own pleasure or 
interest, — proceeding forwards, to the right or left, or even back- 
wards, with longer or shorter pauses, and starting at any particular 
time, — it is obvious that the route taken lies in the man himself and 
is determined by his own peculiar temperament. His judgment, 
experience, and inclination will influence his course at each turn 
of his journey, as new circumstances arise. He will turn aside 
from a mountain which he considers too lofty to be climbed ; he 
will incline to the right, if this direction appears to afford a better 
passage over a swollen stream ; he will rest when he reaches 
a pleasant halting- place, and will hurry on when he knows that 
' See the preceding essay ' On Heredity.' 



138 LIFE AND DEATH, 

enemies beset him. And in spite of the perfectly free choice open 
to him, the course he takes is in fact decided by both the place and 
time of his starting and by circumstances which — always occur- 
ring at every part of the journey — impel him one way or the 
other ; and if all the factors could be ascertained in the minutest 
detail, his course could be predicted from the beginning. 

Such a traveller represents a species, and his route corresponds 
with the changes which are induced in it by natural selection. The 
changes are determined by the physical nature of the species, and 
by the conditions of life by which it is surrounded at any given 
time. A number of different changes may occur at every point, 
but only that one will actually develope which is the most useful, 
under existing external conditions. The species will remain 
unaltered as long as it is in perfect equilibrium with its surround- 
ings, and as soon as this equilibrium is disturbed it will commence 
to change. It may also happen that, in spite of all the pressure 
of competing species, no further change occurs because no one 
of the innumerable very slight changes, which are alone possible 
at any one time, can help in the struggle ; just as the traveller who 
is followed by an overpowering enemy, is compelled to succumb 
when he has been driven down to the sea. A boat alone could 
save him, without it he must perish ; and so it sometimes happens 
that a species can only be saved from destruction by changes of 
a conspicuous kind, and these it is unable to produce. 

And just as the traveller, in the course of his life, can wander an 
unlimited distance from his starting-point, and may take the most 
tortuous and winding route, so the structure of the original 
organism has undergone manifold changes during its terrestrial 
life. And just as the traveller at first doubts whether he will ever 
get beyond the immediate neighbourhood of his starting-point, 
and yet after some years finds himself very far removed from it — 
so the insignificant changes which distinguish the first set of 
generations of an organism lead on through innumerable other 
sets, to forms which seem totally different from the first, but which 
have descended from them by the most gradual transition. All 
this is so obvious that there is hardly any need of a metaphor to 
explain it, and yet it is frequently misunderstood, as shown by the 
assertion that natural selection can create nothing new : the fact 
being that it so adds up and combines the insignificant small de- 



LIFE AND DEATH. 139 

viations presented by natural variation, that it is continually pro- 
ducing something" new. 

If we consider the introduction of natural death in connection 
with the foregoing- statements, we may imagine the process as 
taking place in such a way that, — with the differentiation of Hetero- 
plastids from Homoplastids, and the appearance of division of 
labour among the homogeneous cell-colonies, — natural selection not 
only operated upon the physiological peculiarities of feeding, moving, 
feeling, or reproduction, but also upon the duration of the life of 
single cells. At this developmental stage there would, at any rate, 
be no further necessity for maintaining the power of limitless 
duration. The somatic cells might therefore assume a constitution 
which excluded the possibility of unending life, provided only that 
such a constitution was advantageous for them. 

It may be objected that cells of which the ancestors possessed 
the power of living for ever, could not become potentially mortal 
(that is subject to death from internal causes) either suddenly or 
gradually, for such a change would contradict the supposition which 
attributes immortality to theii* ancestors and to the products of their 
division. This argument is valid, but it only applies so long as 
the descendants retain the original constitution. But as soon as 
the two pi'oducts of the fission of a potentially immortal cell ac- 
quire different constitutions by unequal fission, another possibility 
arises. Now it is conceivable that one of the products of fission 
might preserve the physical constitution necessary for immortality, 
but not the other ; just as it is conceivable that such a cell — 
adapted for unending life — might bud off a small part, which 
would live a long time without the full capabilities of life pos- 
sessed by the parent cell ; again, it is possible that such a cell 
might extrude a certain amount of organic matter (a true excre- 
tion) which is already dead at the moment it leaves the body. 
Thus it is possible that true unequal cell-division, in which only 
one half possesses the condition necessary for increasing, may take 
place ; and in the same way it is conceivable that the constitution 
of a cell determines the fixed duration of its life, examples of 
which are before us in the great number of cells in the higher 
Metazoa, which are destroyed by their functions. The more spe- 
cialized a cell becomes, or in other words, the more it is intrusted 
with only one distinct function, the more likely is this to be the 



140 LIFE AND DEATH. 

case : who then can tell us, whether the limited duration of life was 
brought about in consequence of the restricted functions of the 
cell or whether it was determined by other advantages ^ ? In either 
case we must maintain that the disadvantages arising from a 
limited duration of the cells are more than compensated for by 
the advantages which result from their highly effective specialized 
functions. Although no one of the functions of the body is ne- 
cessarily attended by the limited duration of the cells which per- 
form it, as is proved by the persistence of unicellular forms, yet 
any or all of them might lead to such a limitation of existence 
without in any way injuring the species, as is proved by the 
Metazoa. But the reproductive cells cannot be limited ' in this 
way, and they alone are free from it. They could not lose their 
immoi-tality, if indeed the Metazoa are derived from the immortal 
Protozoa, for from the very nature of that immortality it cannot 
be lost. From this point of view the body, or soma, appears in 
a certain sense as a secondary appendage of the real bearer of 
life, — the reproductive cells. 

Just as it was possible for the specific somatic cells to be differen- 
tiated from among the chemico-physical variations which presented 
themselves in the protoplasm, by means of natural selection, until 
finally each function of the body was performed by its own special 
kind of cell ; so it might be possible for only those variations to 
persist the constitution of which involved a cessation of activity 
after a certain fixed time. If this became true of the whole mass 
of somatic cells, we should then meet with natural death for the 
first time. Whether we ought to regard this limitation of the 
life of the specific somatic cells as a mere consequence of their 
differentiation, or at the same time as a consequence of the powers 
of natural selection especially directed to such an end, — appears 
doubtful. But I am myself rather inclined to take the latter view, 
for if it was advantageous to the somatic cells to preserve the un- 
ending life of their ancestors — the unicellular organisms, this end 

^ The problem is very easily solved if we seek assistance from the principle of 
panmixia developed in the second essay ' On Heredity.' As soon as natural selec- 
tion ceases to operate upon any character, structural or functional, it begins to dis- 
appear. As soon, therefore, as the immortality of somatic cells became useless they 
would begin to lose this attribute. The process would take place more quickly, 
as the histological differentiation of the somatic cells became more useful and com- 
plete, and thus became less compatible with their everlasting duration. — A.W. 1888. 



LIFE AND DEATH. 141 

might have been achieved, just as it was possible at a later period, 
in the higher Metazoa, to prolong' both the duration of life and of 
reproduction a hundred- or a thousand-fold. At any rate, no reason 
can be given which would demonstrate the impossibility of such an 
achievement. 

With our inadequate knowledge it is difficult to surmise the 
immediate causes of such a selective process. Who can point out 
with any feeling of confidence, the direct advantages in which 
somatic cells, capable of limited duration, excelled those capable 
of eternal duration? Perhaps it was in a better performance of 
their special physiological tasks, perhaps in additional material 
and energy available for the reproductive cells as a result of this 
renunciation of the somatic cells ; or perhaps such additional 
power conferred upon the whole organism a greater power of 
resistance in the struggle for existence, than it would have had, 
if it had been necessary to regulate all the cells to a corresponding 
duration. 

But we are not at present able to obtain a clear conception of 
the internal conditions of the organism, especially when we are 
dealing with the lowest Metazoa, which seem to be very rarely 
found at the present day, and of which the vital phenomena we 
only know as they are exhibited by two species, both of doubtful 
origin. Both species have furthermore lost much of their original 
nature, both in structure and function, as a result of their parasitic 
mode of life. Of the Orthonectides and Dicyemidae we know 
something, but of the reproduction in the single free non-parasitic 
form, discovered by F. E. Schulze and named by him Triclioplax 
adhaerens, we know nothing whatever, and of its vital phenomena 
too little to be of any value for the purpose of this essay. 

At this point it is advisable to return once more to the derivation 
of death in the Metazoa from the Orthonectides, as Gotte en- 
deavoured to derive it, when he overlooked the fact that, according 
to his theory, natural death is inherited from the Monoplastids and 
cannot therefore have arisen anew in the Polyplastids. According 
to this theory, death must necessarily have appeared in the lowest 
Metazoa as a result of the extrusion of the germ-cells, and by con- 
tinual repetition must have become hereditary. We must not how- 
ever forget that, in this case, the cause of death is exclusively 
external, by which I mean that the somatic cells which remained 



142 LIFE AND DEATH. 

after the extrusion of the reproductive cells, were unable to feed 
any longer or at any rate to an adequate extent ; and that the cause 
of their death did not lie in their constitution, but in the unfavour- 
able conditions which surrounded them. This is not so much a 
process of natural death as of artificial death, regularly appearing 
in each individual at a corresponding period, because, at a certain 
time of life, the oBganism becomes influenced by the same un- 
favourable conditions. It is just as if the conditions of life in- 
variably led to death by starvation at a certain stage in the life 
of a certain species. But we know that death arises from purely 
internal causes among the higher Metazoa, and that it is antici- 
pated by the whole organisation as the normal end of life. Hence 
nothing is gained by this explanation founded on the Ortho- 
nectides, and we should have to seek further and in a later stage 
of the development of the Metazoa, for the internal causes of true 
natural death. 

Another theory might be based upon the supposition that natural 
death has been derived, in the course of time, from an artificial 
death which always appeared at the same stage of each individual 
life — as we have supposed to be the case in the Orthonectides. I 
cannot agree with this view, because it involves the transmission of 
acquired characters, which is at present unproved and must not be 
assumed to occur until it has been either directly or indirectly de- 
monstrated^. I cannot imagine any way in which the somatic 
cells could communicate this assumed death by starvation to the 
reproductive cells in such a manner that the somatic cells of the 
resulting oiTspring would spontaneously die of hunger in the same 
manner and at a corresponding time as those of the parent. It 
would be as impossible to imagine a theoretical conception of such 
transmission as of the supposed instance of kittens being bom 
without a tail after the parent's tail had been docked ; although 
to make the cases parallel the kittens' tails ought to be lost at the 
same period of life as that at which the parent lost hers. And 
in my opinion we do not add to the intelligibility of such an 
idea by assuming the ai-tificial removal of tails through hundreds 
of generations. Such changes, and indeed all changes, are, as I 
think, only conceivable and indeed possible when they arise from 
within, that is, when they arise from changes in the reproductive 
* See the preceding essay * On Heredity.' 



LIFE AND DEATH. 143 

cells. But I find no difficulty in believing that variations in these 
cells took place during- the transition from Homoplastids to Hetero- 
plastids, variations which formed the material upon which the un- 
ceasing process of natural selection could operate, and thus led to 
the differentiation of the previously identical cells of the colony 
into dissimilar ones — some becoming* perishable somatic cells, and 
others the immortal reproductive cells. 

It is at any rate a delusion to believe that we have explained 
natural death, by deriving it from the starvation of the soma of the 
Orthonectides, by the aid of the unproved assumption of the trans- 
mission of acquired variations. We must first explain why these 
organisms produce only a limited number of reproductive cells 
which are all extruded at once, so that the soma is rendered help- 
less. "Why should not the reproductive cells ripen in succession as 
they do indirectly among the Monoplastides, that is to say in a 
succession of g'enerations, and as they do directly in great num- 
bers among the Metazoa? There would then be no necessity for 
the soma to die, for a few reproductive cells would always be pre- 
sent, and render the persistence of the individual possible. In 
fact, the whole arrangement — the formation of reproductive cells 
at one time only, and their sudden extrusion, — presupposes the 
mortality of the somatic cells, and is an adaptation to it, just as 
this mortality itself must be regarded as an adaptation to the 
simultaneous ripening and sudden extrusion of the generative cells. 
In short, there is no alternative to the supposition stated above, 
viz. that the mortality of the somatic cells arose with the differ- 
entiation of the originally homogeneous cells of the Polyplastids 
into the dissimilar cells of the Heteroplastids. And this is the 
first beginning of natural death. 

Probably at first the somatic cells were not more numerous than 
the reproductive cells, and while this was the case the phenomenon 
of death was inconspicuous, for that which died was very small. 
But as the somatic cells relatively increased, the body became of 
more importance as compared with the reproductive cells, until 
death seems to affect the whole individual, as in the higher 
animals, from which our ideas upon the subject are derived. In 
reality, however, only one part succumbs to natural death, but it is 
a part which in size far surpasses that which remains and is im- 
mortal, — the reproductive cells. 



144 LIFE AND DEATH. 

Gotte combats the statement that the idea of death necessarily 
implies the existence of a corpse. Hence he maintains that the 
cellular sac which is left after the extrusion of the reproductive 
cells among the Orthonectides, and which ultimately dies, is not 
a corpse ; ' for it does not represent the whole organism, any 
more than the isolated ectoderm of any other Heteroplastid ' 
(I.e., p. 48). But it is only a popular notion that a corpse must re- 
present the entire organism. In cases of violent death this idea 
is correct, because then the reproductive cells are also killed. But 
as soon as we recognise that the reproductive cells on the one side, 
and the somatic cells on the other, form respectively the immortal 
and mortal parts of the Metazoan organism, then we must acknow- 
ledge that only the latter, — that is, the soma without the re- 
productive cells, — suffers natural death. The fact that all the 
reproductive cells have not left the body (as sometimes happens) 
before natm-al death takes place, does not affect this conception. 
Among insects, for instance, it may happen that natural death 
occurs before all the reproductive cells have matured, and these 
latter then die with the soma. But this does not make any differ- 
ence to their potential immortality, any more than it modifies the 
scientific conception of a corpse. The idea of natural death in- 
volves that of a corpse, which consists jof the soma, and when the 
latter happens to contain reproductive cells, these do not succumb 
to a natural death, which can never apply to them, but to an acci- 
dental death. They are killed by the death of the soma just as 
they might be killed by any other accidental cause of death. 

The scientific conception of a coi*pse is not affected, whether the 
dead soma remains whole for some time, or falls to pieces at once. 
I cannot therefore agree with Gotte when he denies that an Ortho- 
nectid possesses ' the possibility of becoming a corpse ' (in his sense 
of the word) because ' its death consists in the dissolution of the 
structure of the organism.' When the young of the Khabdites 
form of Ascaris nigrovenosa bore through the body-walls of their 
parent, cause it to disintegrate and finally devour it, the whole 
organism disappears, and it would be difficult to say whether a 
corpse exists in the popular sense of the word. But, scientifically 
speaking, there is certainly a corpse ; the real soma of the animal 
dies, and this, however subdivided, must be considered as a corpse. 
The fact that natural death is so diflicult to define without anv 



LIFE AND DEATH. 145 

accurate conception of what is meant by a corpse, proves the neces- 
sity for arriving at a scientific idea as to the meaning- of the latter. 
There is no death without a corpse — whether the latter be small 
or large, whole or in pieces. 

If we compare the bodies of the higher Metazoa with those of 
the lower, we see at once that not only has the structure of the 
body increased in size and complexity as far as the sovna is con- 
cerned, but we also see that another factor has been introduced, 
which exercises a most important influence in lengthening the 
duration of life. This is the replacement of cells by multipli- 
cation. Somatic cells have acquired (at any rate in most tissues) 
the power of multiplying, after the body is completely developed 
from the reproductive cells. The cells which have undergone 
histological differentiation can increase by fission, and thus supply 
the place of those which are being continually destroyed in the 
course of metabolism. The difference between the higher and 
lower Metazoa in this respect lies in the fact that there is only 
one generation of somatic cells in the latter, and these are used 
up in the process of metabolism at almost the same time that the 
reproductive cells are extruded, while among the former there are 
successive generations of somatic cells. I have elsewhere en- 
deavom-ed to render the duration of life in the animal kingdom 
intelligible by the application of this principle, and have attempted 
to show that its varying duration is determined in different species 
by the varying number of somatic cell-generations^. Of course, 
the varying duration of each cell-generation materially influences 
the total length of life, and experience teaches us that the duration 
of cell-generations varies, not only in the lowest Metazoa as com- 
pared with the highest, but even in the various kinds of cells 
in one and the same species of animal. 

We must, for the present, leave unanswered the question — upon 
what changes in the physical constitution of protoplasm does 
the variation in the capacity for cell-duration depend ; and what 
are the causes which determine the greater or smaller number of 
cell-generations. I mention this obvious difficulty because it is 
the custom to meet every attempt to search deeper into the com- 
mo.n phenomena of life with the reproach that so much is still 
left unexplained. If we must wait for the explanation of these 

* . See the first essay on ' The Duration of Life.' 
L 



146 LIFE AND DEATH. 

processes until we have ascertained the molecular structui-e of cells, 
together with the changes that occur in this structure and the con- 
sequences of the changes, we shall probably never understand either 
the one or the other. The complex processes of life can only be 
followed by degrees, and we can only hope to solve the great 
problem by attacking it from all sides. 

Therefore it is, in my opinion, an advance if we may assume that 
length of life is dependent upon the number of generations of 
somatic cells which can succeed one another in the course of a 
single life ; and, furthermore, that this number, as well as the 
dm-ation of each single cell-generation, is predestined in the germ 
itself. This view seems to me to derive support from the obvious 
fact that the duration of each cell-generation, and also the number 
of generations, undergo considerable increase as we pass from the 
lowest to the highest Metazoa. 

In an earlier work ^ I have attempted to show how exactly the 
duration of life is adapted to the conditions by which it is sur- 
rounded ; how it is lengthened or shortened dm'ing the formation 
of species, according to the conditions of life in each of them ; in 
short, how it is throughout an adaptation to these conditions. A 
few points however were not touched upon in the work referred 
to, and these require discussion ; their consideration will also throw 
some light upon the origin of natural death and the forms of life 
affected by it. 

I have above explained the limited dm-ation of the life of 
somatic cells in the lower Metazoa — Orthonectides — as a pheno- 
menon of adaptation, and have ascribed it to the operation of 
natural selection, at the same time pointing out that the existence 
of immortal Metazoan organisms is conceivable. If the Mono- 
plastides are able to multiply by fission, through all time, then their 
descendants, in which division of labour has induced the antithesis 
of reproductive and somatic cells, might have done the same. If 
the Homoplastid cells reproduced their kind uninterruptedly, equal 
powers of duration must have been possible for the two kinds of 
Heteroplastid cells ; they too might have been immortal so far as im- 
mortality only depends upon the capacity for unlimited reproduction. 

But the capacity for existence possessed by any species is nob 
only dependent upon the power within it; it is also influenced 
^ See the first essay on ' The Duration of Life.' 



IvIPE AND DEATH. 147 

by the conditions of the external world, and this renders neces- 
sary the process which we call adaptation. Thus it is just as in- 
conceivable that either a homogeneous or a heterogeneous cell-colony 
possessing" the physiological value of a multicellular individual 
should continue to grow to an unlimited extent by continued cell- 
division, as it is inconceivable that a unicellular being should 
increase in size to an unlimited extent. In the latter case the 
process of cell-division imposes a limit upon the size attained by 
growth. In the former, the requirements of nutrition, respiration, 
and movement must prescribe a limit to the growth of the cell- 
colony which constitutes the individual of the higher species, just 
as in the case of the unicellular Monoplastides, and it does not 
affect the argument if we consider this limitation to be governed 
by the process of natural selection. It would only be possible to 
regulate the relations of the single cells of the colony to each other 
by fixing the number of cells within narrow limits. During the 
development of MagospJiaera — one of the Homoplastides — the cells 
arrange themselves in the form of a hollow sphere, lying in a 
gelatinous envelope. But the fact that reproduction does not follow 
the simple unvarying rhythm of unicellular organisms is of more 
importance ; for a rhythm of a higher order appears, in which each 
cell of the colony separates from its neighbours, when it has 
reached a certain size, and proceeds by very rapid successive 
divisions to give rise to a certain number of parts which ari'ange 
themselves as a new colony. The number of divisions is controlled 
b}^ the number of cells to which the colony is limited, and at first 
this number may have been very small. With the introduction of 
this secondary higher rhythm during reproduction, the first germ 
of the Polyplastides became evident ; for then each process of fission 
was not, as in unicellular organisms, equivalent to all the others ; 
for in a colony of ten cells the first fission differs from the second, 
third, or tenth, both in the size of the products of division and also 
in remoteness from the end of the process. This secondary fission 
is what we know as segmentation. 

It seems to me of little importance whether the first ' process of 
segmentation takes place in the water or within a cyst, although it 
is quite possible that the necessity for some protective structure 
appeared at a very early period, in order to shield the segmenting 
cell from danger. 

L 2 



148 LIFE AND DEATH. 

It is impossible to accept Gotte's conception of the germ (Keim), 
and at this point the question arises as to its true meaning. I 
should propose to include under this term every cell, cytode, or 
group of cells which, while not possessing the stracture of the 
mature individual of the species, possesses the power of developing 
into it under certain circumstances. The emphasis is now laid 
upon the expression development, which is something opposed to 
simple growth, without change of form. A cell which becomes 
a complete individual by growth alone is not a germ but an 
individual, although a very small one. For example, the small 
encapsuled Heliozoon, which arises as the product of multiple 
fission, is not a germ in our sense of the word. It is an individual, 
provided with all the characteristic marks of its species, and it has 
only to protrude the retracted processes (pseudopodia) and to take 
in the expelled water (formation of vacuoles) in order to become 
capable of living in a free state. In this sense of the word, germs 
are not confined to the Polyplastides, but are found in many Mono- 
plastides. There is nevertheless, in my opinion, a profound and 
significant difference between the germs of these two groups. And 
this lies not so much in the morphological as in the develop- 
mental significance of these structures. As far as I have been able 
to compare the facts, I may state that the germs of the Mono- 
plastides are entirely of secondary origin, and have never formed 
the phyletic origin of the species in ^hich they are found. For 
instance, the spore-formation of the Gregarines resulted from a 
gradually increasing process of division, which was concentrated 
into the period of encystment ; and it was induced by a necessity 
for rapid multiplication due to the parasitic life and unfavourable 
surroundings of these animals. If Gregarines were free-living 
animals, they would not need this method of reproduction. The 
encysted animal would probably divide into eight, four, or two 
parts, or perhaps, like many Infusoria ^, it would not divide at all, 

' These assumptions can be authenticated among the Infusoria. The encysted 
Colpoda cucullus, Ehrbg. divides into two, four, eight, or sixteen parts ; Otostoma 
Carteri, into two, four, or eight ; Tillina magna, Gruber, into four or five ; Lagynxis 
sp. Gruber, into two ; Amphileptus mdeagris, Ehrbg. into two or four. The last two 
species and many others frequently do not divide at all during the encysted con- 
dition. But while any further increase in the number of divisions within the cyst 
does not occur in free-swimming Infusoria, the interesting case of Ichihyophthirius 
multifiliis, Fouquet, shows that parasitic habits call forth a remarkable increase in 



LIFE AND DEATH. 149 

SO that the whole reproduction would depend on simple fission 
alone during" the free state. 

The original mode of reproduction among the Monoplastides was 
undoubtedly simple fission. This became connected with encyst- 
ment, which originally took place without multiplication ; and only 
when the divisions in the cyst became excessively numerous did 
such minute plastids appear that a genuine process of development 
had to be undergone in order to produce complete individuals. 
Here we have the general conception of the germ as I defined it. 
Its limitations are naturally not very sharply defined, for it is 
impossible to draw an absolute distinction between simple growth 
and true development accompanied by changes in form and 
structure. For instance, Hackel's Frotomyxa aurantiaca divides 
within its cyst into numerous plastids, which might be spoken of 
as germs. But the changes of form which they undergo before 
they become young ProtomT/xae are very small, and for the most 
part depend upon the expansion of the body, which existed in the 
capsule as a contracted pear-shaped mass. It is therefore more 
correct to speak only of the simple growth of the products of the 
fission of the parent organism, and to look upon these products 
as young Protomi/xae rather than germs. On the other hand, the 
young animals which creep out of the germs (the ' spores ') of 
Gregarina gigantea, described by E. van Beneden, differ essentially 
from the adult, and pass through a series of developmental stages 
before they assume the characteristic form of a Gregarine. 

This is true development ^. But such a method of germ-formation 
and development are found most frequently, although not ex- 
clusively, among the parasitic Monoplastides, and this fact alone 
serves to indicate their secondary origin. It is a form of ontogenetic 
development differing from that of the Polyplastides in that it does 
not revert to a phyletically primitive condition of the species, but, 
on the contrary, exhibits stages which first aj)pear in the phyletic 

the number of divisions. This animal divides into at least a thousand daughter in- 
dividuals. 

^ True development also takes place in the above-mentioned IchthyopJithirius. 
"While in other Infusoria the products of fission exactly resemble the parent, in 
IchtliyopMhirius they have a different form ; the sucking mouth is wanting while 
provisional clasping cilia are at first present. In this case therefore the word germ 
may be rightly applied, and Ichthyojphthirius affords an interesting example of the 
phyletic origin of germs among the lower Flagellata and Gregarines. Cf. Fouquet, 
'Arch. Zool. Experimentale,' Tom. V. p. 159. 1876. 



150 LIFE AND DEATH, 

development of the specific form. The Psorosperms were only 
formed after the Gregfarines had become established as a group. The 
amoeboid organisms which creep out of them are in no way to be 
regarded as the primitive forms of the Gregarines, even if the 
latter may have resembled them, but they are coenogenetic forms 
produced by the necessity for a production of numerous and very 
minute germs. The necessity for a process of genuine develop- 
ment perhaps depends upon the small amount of material contained 
in one of these germs, and on other conditions, such as change of 
host, change of medium, etc. It therefore results that the funda- 
mental law of biogenesis does not apply to the Monoplastides ; for 
these forms are either entirely without a genuine ontogeAy and 
only possess the possibility of growth, or else they are only endowed 
with a coenogenetic ontogeny ^. 

Some authorities may be inclined to limit the above proposition, 
and to maintain that we must admit the possibility that we are 
likely to occasionally meet with an ontogeny of which the stages 
largely correspond with the most important stages in the phyletic 
development of the species, and that the ontogenetic repetition of 
the phylogeny, although not the rule, may still occur as a rare 
exception in the Protozoa. 

A careful consideration of the subject indicates, however, that 
the occurrence of such an exception is very improbable. Such an 
ontogeny would, for instance, occur if one of the lowest Mono- 
plastides, such as a Moneron, were to develope into a higher form, 
such as one of the Flagellata, with mouth, eye-spot, and cortical 
layer, under such external conditions that it would be advantageous 
for the existence of its species that it should no longer reproduce 
itself by simple fission, but that the periodical formation of a cyst 
(which was perhaps previously part of the life-history) should be 
associated with the occurrence of numerous divisions within the cyst 
itself, and with the formation of germs. We must suppose either 
that these germs were so minute that the young animals could not 

* Butschli, long ago, doubted the application of tlie fundamental law of bioge- 
nesis to the Protozoa (cf. ' Ueber die Entstehung der Schwarmsprosslings der Podo- 
phrya quadripartita,' Jen. Zeit. f. Med. u. Naturw. Bd. X. p. 19, Note). Gruber haa 
more recently expressed similar views, and in fact denies the presence of develop- 
ment in the Protozoa, and only recognizes growth (' Dimorpha mutans, Z. f. W. Z.' Bd. 
XXXVII. p. 445). This proposition must however be restricted, inasmuch as a de- 
velopment certainly occurs, although one which is coenogenetic and not palingenetic. 



LIFE AND DEATH. 151 

become Flagellata directly, or that it was advantageous for them to 
move and feed as Mgnera at an early period, and to assume the 
more complex structure of the parent by gradual stages. In other 
words, the phyletic development would proceed hand in hand with 
the ontogeny corresponding to it, although not from any in- 
ternal cause, but as an adaptation to the existing conditions of 
life. But the supposed transformation of the species also depended 
upon these same conditions of life, which must therefore have been 
of such a natuj'e as to bring about simultaneously, by an inter- 
calation of germs and by a genuine development, the evolution of 
the form in question in the last stage of its ontogeny, and the 
maintenance of its original condition during the initial stage. 
Such a combination of circumstances can have scarcely ever 
happened. Against the occurrence of such a transformation as we 
have supposed, it might be argued, indeed, that the assumed pro- 
duction of very numerous germs does not occur among free-living 
Monoplastides. Those which have acquired parasitic habits must be 
younger phyletic forms, for their first host — whether a lowly or 
a highly organized Metazoon — must have appeared before they 
could gain access to it and adapt themselves to the conditions of 
a parasitic life, and by this time the Flagellate Infusoria were 
already established. It is by far less probable that the persistence 
or rather the intercalation of the ancestral form would occur in an 
ontogenetic cycle, consisting of a series of stages, and not of 
two only, as in our example. For as soon as reproduction can be 
effected by the simple fission of the adult, not only is there no 
reason why the earlier phyletic stages should be again and 
again repeated, but such recapitulation is simply impossible. 
We cannot, therefore, conclude that the anomalous early stages of 
a Monoplastid such as Acineta correspond with an early form of 
phyletic development. 

Supposing, for instance, that the Acinetaria were derived from 
the Ciliata, then this transformation must have taken place in the 
course of the continued division of the ciliate ancestor — partially 
connected with encystment, but for the most part independently of 
it. Of the myriads of generations which such a process of develop- 
ment may have occupied, perhaps the first set moved with suctorial 
processes, while the second gradually adopted sedentary habits, and 
throughout the whole of the long series, each succeeding generation 



152 LIFE AND DEATH. 

must have been almost exactly like its predecessor, and must 
always have consisted of individuals which possessed the characters 
of the species. 

This does not exclude the possibility that in spite of an assumed 
sedentary mode of life, the need for locomotion and for obtaining 
food in fresh places may have arisen at some period of life. But 
whenever formation of swarm-spores takes place instead of simple 
fission, this does not depend upon the persistence of an ancestral 
form in the ontogenetic cycle, but is due to the intercalation of an 
entirely new ontogenetic stage, which happens to resemble an 
ancestral form, in the possession of cilia, etc. 

I imagine that I have now sufficiently explained the above 
proposition, that the repetition of the phylogeny in the ontogeny 
does not and cannot occur among unicellular organisms. 

"With the Polyplastides the opposite is the case. There is no 
species, as far as we know, which does not — either in each in- 
dividual, or after long cycles which comprise many individuals 
(alternation of generations) — invariably revert to the INIonoplastid 
state. This applies from the lowest forms, such as Magosphaera and 
the Orthonectides, up to the very highest. In the latter a great 
number of intermediate phyletic stages always occur, although 
some have been omitted as the result of concentration in the 
ontogeny, while others have sometimes been intercalated. 

Sexual reproduction is the obvious cause of this very important 
arrangement. Even if this is an hypothesis rather than a fact 
we must nevertheless accept it unconditionally, because it is a 
method of reproduction found everywhere. It is the rule in every 
group of the animal kingdom, and is only absent in a few species in 
which it is replaced by parthenogenesis. In these latter instances 
sexual reproduction may be local, and entirely absent in certain 
districts only (Ajms), or it may be only apparently wanting ; in some 
cases where it is undoubtedly absent, it is equally certain that it 
was present at an earlier period [Limnadia Hermanni). We cannot 
as yet determine whether its loss will not involve the degeneration 
and ultimate extinction of the species in question. 

If the essential nature of sexual reproduction depends upon the 
conjugation of two equivalent but dissimilar morphological elements, 
then we can understand that a multicellular being can only attain 
sexual reproduction when a unicellular stage is present in its 



LIFE AND DEATH. 153 

development ; for the coalescence of entire multicellular organisms 
in such a manner that fusion would only take place between equi- 
valent cells, would seem to be impracticable. In the necessity 
for sexual reproduction, there is therefore also implied the ne- 
cessity for reverting to the original condition of the Polyplastides — 
that of a single cell — and upon this alone depends the fundamental 
law of biogenesis. This law is therefore confined to the Poly- 
plastides, and does not apply to the Monoplastides ; and Gotte's 
suggestion that the latter fall back into the primitive condition 
of the organism during their encystment (rejuvenescence), finds 
no support in this aspect of the question. 

I have on a previous occasion ^ referred the utility of death to the 
ultimate fact that the unending life of the Metazoan body would 
be a useless luxmy, and to the fact that the individuals would 
necessarily become injured in the course of time, and would be 
therefore ' not only valueless to the species, but . . . even harmful, 
for they take the place of those which are sound ' (1. c, p. 24). I 
might also have said that such damaged individuals would sooner 
or later fall victims to some accidental death, so that there would 
be no possibility of real immortality. I now propose to ex- 
amine this statement a little more closely, and to return to a 
question which has already been alluded to before. 

It is obvious that the advantages above set forth did not form 
the motive which impelled natural selection to convert the im- 
mortal life of the Monoplastides into the life of limited duration 
possessed by the Heteroplastides, or more correctly, which led to the 
restriction of potential immortality to the reproductive cells of the 
latter. It is at any rate theoretically conceivable that a struggle 
might arise between the mortal and immortal individuals of a 
certain Metazoan species, and that natural selection might secure 
the success of the former, because the longer the immortal in- 
dividuals lived, the more defective they became, and as a result gave 
rise to weaker offspring in diminished numbers. Probably no one 
would be bold enough to suggest such a crude example of natural 
selection. And yet I venture to think that the principle of 
natm-al selection is here also to be taken into account, and even 
plays, although in a negative rather than a positive way, a very 
essential part in determining the duration of life in the Metazoa. 
1 See the first essay on 'The Duration of Life,' p. 23 et seq. 



154 LIFE AND DEATH. 

When the somatic cells of the first Heteroplastides ceased to be 
immortal, such a loss would not in any way have precluded them 
from regaining this condition. Just as, with the differentiation of 
the first somatic cells of the lowest Heteroplastides, their duration 
was limited to that of a single cell -generation, — so it must have 
been possible for them, at a later period and if the necessity arose, 
to lengthen their duration over two, three, or more generations. 
And if my theory of the " duration of life in the Metazoa is well 
founded, these cells have as a matter of fact increased their duration, 
to an extent about equal to that of the organism to which they 
belong. There is no ground whatever for the assumption that it 
is impossible to fix the number of cell-generations at infinity, — as 
actually happens in the case of the reproductive cells, — but on the 
other hand it has already been shown to be obvious that such an 
extension is opposed to the principle of utility. It could never be 
to the advantage of a species to produce crippled individuals, and 
therefore the infinite duration of individuals has never reappeared 
among the Metazoa. So far the limited duration of Meta- 
zoan life may be attributed to the worthlessness or even the 
injm'ious nature of individuals, which although immortal, were 
nevertheless liable to wear and tear. This fact explains why im- 
mortality has never reappeared, it explains the predominance of 
death, but it was not the single primary cause of this phenomenon. 
The perishable and vulnerable nature of the soma was the reason 
why nature made no effort to endow this part of the individual 
with a life of unlimited length. 

Gotte considers that death is inherent in reproduction, and in 
a certain sense this is true, but not in the general way supposed by 
him. 

I have endeavoured to show above that it is most advantageous 
for the preservation of the species among the lowest Metazoa, that 
the body should consist of a relatively small number of cells, and 
that the reproductive cells should ripen simultaneously and all 
escape together. If this conclusion be accepted, the uselessness of 
a prolonged life to the somatic cells is obvious, and the occurrence 
of death at the time of the extrusion of the reproductive cells is 
explained. In this manner death (of the soma) and reproduction 
are here made to coincide. 

This relation of reproduction to death still exists in a great num- 



LIFE AND DEATH. 155 

ber of the higher animals. But such an association, tog-ether with 
the simultaneous ripening of the reproductive cells, has not been 
maintained continuously in the past. As the soma becomes larger 
and more highly organized, it is able to withstand more injuries, 
and its average duration of life will extend : pari passu with these 
changes it will become increasingly advantageous not only for the 
number of reproductive cells to be multiplied, but also for the time 
during which they are produced to be prolonged. In this manner 
a lengthening of the reproductive period arises, at first continuously 
and then periodically. It is beyond my present purpose to consider 
in detail the conditions upon which this lengthening depends, but 
I would emphasize the fact that a lengthening of life is connected 
with the increase in the duration of reproduction, while on the 
other hand there is no reason to expect life to be prolonged 
beyond thie reproductive period ; so that the end of this period is 
usually more or less coincident with death. 

A further prolongation of life could only take place when the 
parent begins to undertake the duty of rearing the young. The 
most primitive form of this is found among those animals, which 
do not expel their reproductive cells as soon as they are ripe but 
retain them within their bodies, so that the early stages of develop- 
ment take place under the shelter of the parent organism. Associ- 
ated with such a process there is frequently a necessity for the 
germs to reach a certain spot, where alone their further development 
can take place. Thus a segment of a tapeworm lives until it 
has brought the embryos into a position which affords the possibility 
of their passive transference to the stomach of their special host. 
But the duratjon of life is first materially lengthened when the off- 
spring begin to be really tended, and as a general rule the increase 
in length is exactly proportional to the time which is demanded by 
the care of the young. Accurately conducted observations are 
wanting upon this precise point, but the general tendency of the 
facts, as a whole, cannot be doubted. Those insects of which the 
care for their offspring terminates with the deposition of eggs at the 
appropriate time, place, etc., do not survive this act ; and the dura- 
tion of life in such imagos is shorter or longer according as the 
eggs are laid simultaneously or ripen gradually. On the other hand, 
insects — such as bees and ants — which tend their young, have a life 
which is prolonged for years. 



156 LIFE AND DEATH. 

But the lengthening of the reproductive period alone may result 
in a marked increase in the length of life, as is proved by the queen- 
bee. In all these cases it is easy to imagine the operation of 
natural selection in producing such alterations in the duration of 
life, and indeed we might accurately calculate the amount of in- 
crease which would be produced in any given case if the necessary 
data were available, viz. the physiological strength of the body, and 
its relations to the external world, such as, for instance, the power 
of obtaining food at various periods of life, the expenditure of energy 
necessary for this end, and the statistics of destruction, that is, the 
probabilities in favour of the accidental death of a single individual at 
any given time. These statistics must be known both for the images, 
larvae, and eggs ; for the lower they are for the imagos, and the 
higher for the larvae and eggs, the more advantageous will it be, 
ceteris ^;flr2^?/«, for the number of eggs produced by the imago to 
be increased, and the more probable it would therefore be that a 
long reproductive period, involving a lengthening of the life of the 
imago, would be introduced. But we are still far from being able 
to apply mathematics to the phenomena of life ; the factors are too 
numerous, and no attempt has been made as yet to determine them 
with accuracy. 

But we must at least admit the principle that both the lengthen- 
ing and shortening of life are possible by means of natural selection, 
and that this process is alone able to render intelligible the exact 
adaptation of the length of life to the conditions of existence. 

A shortening of the normal duration of life is also possible ; this 
is shown in every case of sudden death, after the deposition of the 
whole of the eggs at a single time. This occurs among certain 
insects, while nearly allied forms of which the oviposition lasts over 
many days therefore possess a correspondingly long imago-life. The 
Ephemeridae and Lepidoptera afford many examples of this, and in an 
earlier work I have collected some of them ^. The humming-bird 
hawk-moth flies about for weeks laying an egg here and there, and, 
like the allied poplar hawk-moth and lime hawk-moth, probably 
dies when it has deposited all the eggs which can be matured with 
the amount of nutriment at its disposal. Many other Lepidoptera, 
such as the majority of butterflies, fly about for weeks depositing 
their eggs, but others, such as the emperor-moths and lappet- 
' See Appendix to the first essay on ' The Duration of Life,' pp. 43-46. 



LIFE AND DEATH. 157 

moths, lay their egg's one after another and then die. The eggs of 
the parthenogenetic Psi/cJddae are laid directly after the imago has 
left the cocoon, and death ensues immediately, so that the whole life 
of the imago only lasts for a few hours. No one could look upon 
this brief life as a primitive arrangement among Lepidoptera, any 
more than we do upon the absence of wings in the female Psi/chidae-, 
shortening of life here is therefore clearly explicable. 

In such cases have we any right to speak of the fatal effect of 
reproduction ? We may certainly say that these insects die of 
exhaustion ; their vital strength is used up in the last effort of 
laying eggs, and in the case of the males, in the act of copulation. 
Reproduction is here certainly the most apparent cause of death, 
but a more remote and deeper cause is to be found in the limita- 
tion of vital strength to the length and the necessary duties of 
the reproductive period. The fact that there are female Lepi- 
doptera which, like the emperor -moths, do not feed in the 
imago-state, proves the truth of this statement. They still 
possess a mouth and a complete alimentary canal, but they have 
no spiral ' tongue,' and do not take food of any kind, not even a 
drop of water. They live in a torpid condition for days or weeks 
until fertilization is accomplished, and then they lay their eggs and 
die. The habit of extracting honey from flowers — common to most 
hawk-moths and butterflies — would not have thus fallen into 
disuse, if the store of nutriment, accumulated in the form of the fat- 
bodies, during the life of the caterpillar, had not been exactly 
sufficient to maintain life until the completion of oviposition. The 
fact that the habit of taking food has been thus abandoned is a 
proof that the duration of life beyond the reproductive period would 
not be to the advantage of the species. 

The protraction of existence into old age among the higher 
Metazoa proves that death is not a necessary consequence of repro- 
duction. It seems to me that Gotte's statement * that the 
appearances of senility must not be regarded as the general cause 
of death ' is not in opposition to my opinions but rather to those 
which receive general acceptance. I have myself pointed out that 
' death is not always preceded by senility or a period of old 
age ^.' 

The materials are wanting for a comprehensive investigation of 

* See the first essay on ' The Duration of Life,' p. 21. 



]58 LIFE AND DEATH. 

the causes which first introduced this period among the higher 
Metazoa ; in fact the most fundamental data are absent, for we do 
not even know the part of the animal kingdom in which it first 
appeared : we cannot even state the amount by which the duration 
of life exceeds that of the period of reproduction, or what is the 
value to the species of this last stage in the life of the individual. 

It is in these general directions that we must seek for the sig- 
nificance of old age. It is obviously of use to man, for it enables 
the old to care for their children, and is also advantageous in enabling 
the older individuals to participate in human afiairs and to exer- 
cise an influence upon the advancement of intellectual powers, and 
thus to influence indirectly the maintenance of the race. But as 
soon as we descend a step lower, if only as far as the apes, accurate 
facts are wanting, for we are, and shall probably long be, ignorant 
of the total duration of their life, and the point at which the period 
of reproduction ceases. 

I must here break off in the midst of these considerations, rather 
than conclude them, for much still remains to be said. I hope, 
nevertheless, that I have thrown new light upon some important 
points, and I now propose to conclude with the following short 
abstract of the results of my enquiry. 

I. Natural death occurs only among multicellular beings ; it 
is not found among unicellular organisms. The process of encyst- 
ment in the latter is in no way comparable with death. 

II. Natural death first appears among the lowest Heteroplastid 
Metazoa, in the limitation of all the cells collectively to one 
generation, and of the somatic or body-cells proper to a restricted 
period : the somatic cells afterwards in the higher Metazoa came to 
last several and even many generations, and life was lengthened to 
a corresponding degree. 

III. This limitation went hand in hand with a differentiation 
of the cells of the organism into reproductive and somatic cells, 
in accordance with the principle of division of labour. This diffe- 
rentiation took place by the operation of natural selection. 

IV. The fundamental biogenetic law applies only to multi- 
cellular beings; it does not apply to unicellular forms of life. 
This depends on the one hand upon the mode of reproduction by 
fission which obtains among the Monoplastides (unicellular or- 



LIFE AND DEATH. 159 

ganisms), and on the other upon the necessity, induced by sexual 
reproduction, for the maintenance of a unicellular stag-e in the 
development of the Polyplastides (multicellular org-anisms). 

V. Death itself, and the longer or shorter duration of life, 
both depend entirely on adaptation. Death is not an essential 
attribute of living matter ; it is neither necessarily associated with 
reproduction, nor a necessary consequence of it. 

In conclusion, I should wish to call attention to an idea which i5 
rather implied than expressed in this essay : — it is, that reproduc- 
tion did not first make its appearance coincidently with death. 
Reproduction is in truth an essential attribute of living matter, just 
as is the growth which gives rise to it. It is as impossible to 
imagine life enduring without reproduction as it would be to 
conceive life lasting* without the capacity for absorption of food 
and without the power of metabolism. Life is continuous and 
not periodically interrupted : ever since its first appearance upon 
the earth, in the lowest organisms, it has continued without break ; 
the forms in which it is manifested have alone undergone change. 
Every individual alive to day — even the very highest — is to be 
derived in an unbroken line from the first and lowest forms. 



>y 



/^/ 



IV. 



THE CONTINUITY OF THE GEEI-PIASI AS THE 
FOUNDATION OF A THEOEY OF HEEEDITY. 

1885. 



M 



CONTINUITY OF THE GERM-PLASM, &c. 



PREFACE. 

The ideas developed in this essay were first expressed during 
the past winter in a lecture delivered to the students of this Uni- 
versity (Freiburg"), and they were shortly afterwards — in February 
and the beginning- of March — written in their present form. 
I mention this, because I might otherwise be reproached for a 
somewhat partial use of the most recent publications on related 
subjects. Thus I did not receive Oscar Hertwig's paper — ' Contri- 
butions to the Theory of Heredity ' (Zur Theorie der Vererbung), 
until after I had finished writing my essay, and I could not there- 
fore make as much use of it as I should otherwise have done. 
Furthermore, the paper by Kolliker on ' The Significance of the 
Nucleus in the Phenomena of Heredity ' (Die Bedeutung der Zell- 
keme fiir die Vorgange der Vererbung), did not appear until after 
the completion of my manuscript. The essential treatment of the 
subject would not, however, have been altered if I had received the 
papers at an earlier date, for as far as the most important point — the 
significance of the nucleus — is concerned, my views are in accord- 
ance with those of both the above-named investigators ; while the 
points upon which our views do not coincide had already received 

attention in the manuscript. 

AW. 

Fbeibdrg I. Bretsgau, 
June i6, 1885. 



/c^ 



CONTINUITY OF THE GERM-PLASM, &c. 



CONTENTS. 



PAOtS 



Introduction 165 

I. The Gebm-plasm 174 

1. Historical development of the theory as to the localization of the germ- 

plasm in the nucleus . . . . . . . . .174 

2. Nageli's 'idioplasm' is not identical with Weismann's ' genn-plasm ' . 180 

3. A retransformation of somatic idioplasm into germ-idioplasm does not 

take place ........... 183 

4. Confirmation of the theory as to the significance of the nuclear sub- 

stance afforded by Nussbaum's and Gruber's experiments on re- 
generation in Infusoria ......... 185 

5. The nucleoplasm changes during ontogeny according to a certain law . 186 

6. The identity of the daughter-nuclei produced by indirect nuclear 

division, as assumed by Strasburger, is not necessary for my theory 187 
7- The gradual decrease in complexity of the structure of the nucleus. 

during ontogeny . . . . . . . . . . 190 

8. Nageli's view on the germs (' Anlagen') in the idioplasm . . . 192 

9. The manner in which germ-cells arise from somatic cells . . . 194 

10. ' Embryonic ' cells in the mature organism ...... 196 

11. The rule of probability is against a retransformation of somatic idio- 

plasm into germ-plasm . . . . . . . . .198 

12. The regular cyclical development of the idioplasm founded upon 

phylogeny by Nageli . . . . . . . . .199 

13. It follows from phyletic considerations that the germ-cells have not 

arisen at the end of ontogeny ....... 201 

14. They originally arose at the beginning of ontogeny, but at a later 

period the time of their origin was displaced ..... 202 

15. A continuity of the germ-cells does not now exist in most cases . . 205 

16. But there is a continuity of the gevm.-plasm ...... 205 

17. Strasburger's objection to my supposition that the germ-plasm passes 

along distinct routes ......... 209 

18. The cell-body may remain unchanged when the nucleus is changed . 210 

19. It is conceivable that all somatic nuclei may contain some germ-plasm 211 

II. The SIGNIFICANCE OF THE PoLAB Bodies 212 

1. The egg-cell contains two kinds of idioplasm ; germ-plasm and histo- 

genetic nucleoplasm . . . . . . . . . .213 

2. The expulsion of the polar bodies signifies the removal of the histo- 

genetic nucleoplasm . . . . . . . . . .214 

3. Other theories as to the significance of the polar bodies . . .214 

M 2, 



164 CONTINUITY OF THE GERM-PLASM, &C. — CONTENTS. 

PAGE 

4. The modes of occurrence of polar bodies 217 

5. Their possible occurrence in male germ-cells 219 

6. There are two kinds of nucleoplasm in the male germ-cells . . .219 

7. Polar bodies in plants 222 

8. Morphological origin of polar bodies aaj 

III. On the nature of Parthenogenesis 225 

1. The phenomena exhibited in the maturation of the egg are identical 

in parthenogenetic and sexual development . . . . .225 

2. The difference between parthenogenetic and sexual cells must be of a 

quantitative nature . . . . . . . . . .226 

3. The quantity of the germ-plasm determines development . . . 227 

4. The expulsion of polar bodies depends upon the antagonism between 

germ-plasm and ovogenetic nucleoplasm 230 

, 5. Fertilization does not act dynamically 231 

6. An insufficient quantity of germ-plasm arrests development . . 232 

7. Relation of the nucleus to the cell ....... 234 

8. The case of the bee does not constitute any objection to my theory . 234 

9. Strasburger's views upon parthenogenesis 237 

10. Parthenogenesis does not depend upon abundant nutrition . . .339 

11. The indirect causes of sexual and parthenogenetic reproduction . . 241 

12. The direct causes 242 

13. Explanation of the formation of nutritive cells 243 

14. Identity of the germ-plasm in male and female germ-cells . . . 246 

Note 249 



IV. 



THE CONTINUITY OF THE GERM-PLASM AS THE 
FOUNDATION OF A THEORY OF HEREDITY. 

Introduction. 

When we see that, in the higher organisms, the smallest 
structural details, and the most minute peculiarities of bodily and 
mental disposition, are transmitted from one generation to another ; 
when we find in all species of plants and animals a thousand 
characteristic peculiarities of structure continued unchanged through 
long series of generations ; when we even see them in many cases 
unchanged throughout whole geological periods ; we very naturally 
ask for the causes of such a striking phenomenon : and enquire how 
it is that such facts become possible, how it is that the individual is 
able to transmit its structural features to its offspring with such 
precision. And the immediate answer to such a question must be 
given in the following terms : — ' A single cell out of the millions 
of diversely differentiated cells which compose the body, becomes 
specialized as a sexual cell ; it is thrown off from the organism 
and is capable of /reproducing all the peculiarities of the parent 
body, in the new individual which springs from it by cell-division 
and the complex process of differentiation.' Then the more precise 
question follows : ' How is it that such a single cell can reproduce 
the tout ensemble of the parent with all the faithfulness of a 
portrait ? ' 

The answer is extremely difficult ; and no one of the many 
attempts to solve the problem can be looked upon as satisfactory ; 
no one of them can be regarded as even the beginning of a solution or 
as a secure foundation from which a complete solution may be 
expected in the future. Neither Hackel's^, 'Perigenesis of the 
Plastidule,' nor Darwin's ^ ' Pangenesis,' can be regarded as such a 
beginning. The former hypothesis does not really treat of that 

^ Hackel, 'Ueber die Wellenzeugung der Lebenstheilchen etc..' Berlin, 1876. 
' Darwin, ' The Variation of Animals and Plants under Domestication,' vol. ii. 
1875, chap, xxvii. pp. 344-399- 



166 THE CONTINUITY OF THE GERM-PLASM AS THE 

part of the problem which is here placed in the foreground, viz. 
the explanation of the fact that the tendencies of heredity are 
present in single cells, but it is rather concerned with the question 
as to the manner in which it is possible to conceive the trans- 
mission of a certain tendency of development into the sexual cell, 
and ultimately into the organism arising from it. The same may 
be said of the hypothesis of His ^, who, like Haekel, regards heredity 
as the transmission of certain kinds of motion. On the other hand, 
it must be conceded that Darwin's hypothesis goes to the very root 
of the question, but he is content to give, as it were, a provisional 
or purely formal solution, which, as he himself says, does not claim 
to afford insight into the real phenomena, but only to give us 
the opportunity of looking at all the facts of heredity from a 
common standpoint. It has achieved this end, and I believe it 
has unconsciously done more, in that the thoroughly logical ap- 
plication of its principles has shown that the real causes of 
heredity cannot lie in the formation of gemmules or in any 
allied phenomena. The improbabilities to which any such theory 
would lead are so great that we can affirm with certainty 
that its details cannot accord with existing facts. Further- 
more, Brooks' ^ well-considered and brilliant attempt to modify 
the theory of Pangenesis, cannot escape the reproach that it 
is based upon possibilities, which one might certainly describe as 
improbabilities. But although I am of opinion that the whole 
foundation of the theory of Pangenesis, however it may be modified, 
must be abandoned, I think, nevertheless, its author deserves 
great credit, and that its production has been one of those indirect 
roads along which science has been compelled to travel in order to 
arrive at the truth. Pangenesis is a modem revival of the oldest 
theory of heredity, that of Democritus, according to which the 
sperm is secreted from all parts of the body of both sexes during 
copulation, and is animated by a bodily force ; according to this 
theory also, the sperm from each part of the body .reproduces the 
same part ^. 

' His, ' Unsre Korperform etc.,' Leipzig, 1875. 

' Brooks, 'The Law of Heredity,' Baltimore, 1883. 

' Galton'8 experiments on transfusion in Rabbits have in the mean time really 
proved that Darwin's gemmules do not exist. Roth indeed states that Darwin has 
never maintained that his gemmules make use of the circulation as a medium, but 
while on the one hand it cannot be shown why they should fail to take the 



FOUNDATION OF A THEOEY OF HEEEDITY. 167 

If, according to the received physiological and morphological 
ideas of the day, it is impossible to imagine that gemmules pro- 
duced by each cell of the organism are at all times to be found in 
all parts of the body, and furthermore that these gemmules are col- 
lected in the sexual cells, which are then able to again reproduce in 
a certain order each separate cell of the organism, so that each 
sexual cell is capable of developing into the likeness of the parent 
body ; if all this is inconceivable, we must enquire for some other 
way in which we can arrive at a foundation for the true under- 
standing of heredity. My present task is not to deal with the 
whole question of heredity, but only with the single although 
fundamental question — * How is it that a single cell of the body 
can contain within itself all the hereditary tendencies of the whole 
organism ? ' I am here leaving out of account the further ques- 
tion as to the forces and the mechanism by which these ten- 
dencies are developed in the building-up of the organism. On 
this account I abstain from considering at present the views of 
Nageli, for as will be shown later on, they only slightly touch this 
fundamental question, although they may certainly claim to be of 
the highest importance with respect to the further question alluded 
to above. 

Now if it is impossible for the germ-cell to be, as it were, 
an extract of the whole body, and for all the cells of the organism 
to despatch small particles to the germ-cells, from which the 
latter derive their power of heredity; then there remain, as it 
seems to me, only two other possible, physiologically conceivable, 
theories as to the origin of germ-cells, manifesting such powers as 
we know they possess. Either the substance of the parent germ- 
cell is capable of undergoing a series of changes which, after the 
building-up of a new individual, leads back again to identical germ- 
cells ; or the germ-cells are not derived at all, as far as their 
essential and characteristic substance is concerned, from the body of 

favourable opportunities afforded by such a medium, inasmuch as they are said to be 
constantly circulating through tlie body ; so on the other hand we cannot understand 
how the gemmules could contrive to avoid the circulation. Darwin has acted very 
wisely in avoiding any explanation of the exact course in which his gemmules 
circulate. He offered his hypothesis as a formal and not as a real explanation. 

Professor Meldola points out to me that Darwin did not admit that Galton's ex- 
periments disproved pangenesis (* Nature,' April 27, 1871, p. 502), and Galton also 
admitted this in the next number of 'Nature ' (May 4, 1871, p. 5). — A. W. 1889. 



168 THE CONTINUITY OF THE QEEM-PLASM AS THE 

the individual, but they axe derived directly from the parent germ- 
cell. 

I believe that the latter view is the true one : I have expounded 
it for a number of years, and have attempted to defend it, and to 
work out its further details in various publications. I propose to 
call it the theory of ' The Continuity of the Germ-plasm,' for it is 
founded upon the idea that heredity is brought about by the trans- 
ference from one generation to another, of a substance with a defi- 
nite chemical, and above all, molecular constitution. I have called 
this substance ' germ-plasm,' and have assumed that it possesses 
a highly complex structure, conferring upon it the power of de- 
veloping into a complex organism. I have attempted to explain 
heredity by supposing that in each ontogeny, a part of the specific 
germ-plasm contained in the parent egg-cell is not used up in the 
construction of the body of the offspring, but is reserved unchanged 
for the formation of the germ-cells of the following generation. 

It is clear that this view of the origin of germ-cells explains the 
phenomena of heredity very simply, inasmuch as heredity becomes 
thus a question of growth and of assimilation, — the most funda- 
mental of all vital phenomena. If the germ-cells of successive 
generations are directly continuous, and thus only form, as it were, 
different parts of the same substance, it follows that these cells 
must, or at any rate may, possess the same molecular constitution, 
and that they would therefore pass through exactly the same stages 
under certain conditions of development, and would form the same 
final product. The hypothesis of the continuity of the germ-plasm 
gives an identical starting-point to each successive generation, and 
thus explains how it is that an identical product arises from all of 
them. In other words, the hypothesis explains heredity as part of 
the underlying problems of assimilation and of the causes which act 
directly during ontogeny: it therefore builds a foundation from 
which the explanation of these phenomena can be attempted. 

It is true that this theory also meets with difficulties, for it 
seems to be unable to do justice to a certain class of phenomena, viz. 
the transmission of so-called acquired characters. I therefore gave 
immediate and special attention to this point in my first publi- 
cation on heredity ^, and I believe that I have shown that the 

^ Weismann, ' Ueber die Vftrerbung.' Jena, 1883; translated in the present volume 
as the second essay ' On Heredity.' 



FOUNDATION OB"" A THEORY OF HEEEDITY. 169 

hypothesis of the transmission of acquired characters — up to that 
time generally accepted — is, to say the least, very far from being" 
proved, and that entire classes of facts which have been interpreted 
under this hypothesis may be quite as well interpreted otherwise, 
while in many cases they must be explained differently. I have 
shown that there is no ascertained fact, which, at least up to the 
present time, remains in irrevocable conflict with the hypothesis of 
the continuity of the germ-plasm ; and I do not know any reason 
why I should modify this opinion to-day, for I have not heard of 
any objection which appears to be feasible. E. Roth ^ has objected 
that in pathology we everywhere meet with the fact that acquired 
local disease may be transmitted to the offspring as a predispo- 
sition ; but all such cases are exposed to the serious criticism that 
the very point that first needs to be placed on a secure footing is 
incapable of proof, viz. the hypothesis that the causes which in each 
particular case led to the predisposition, were really acquired. 
It is not my intention, on the present occasion, to enter fully 
into the question of acquired characters ; I hope to be able to 
consider the subject in greater detail at a future date. But in 
the meantime I should wish to point out that we ought, above 
all, to be clear as to what we really mean by the expression ' ac- 
quired character.' An organism cannot acquire anything unless it 
already possesses the predisposition to acquire it : acquired cha- 
racters are therefore no more than local or sometimes general 
variations which arise under the stimulus provided by certain ex- 
ternal influences. If by the long-continued handling of a rifle, the 
so-called ' Exercierknochen ' (a bony growth caused by the pres- 
sure of the weapon in drilling) is developed, such a result depends 
upon the fact that the bone in question, like every other bone, con- 
tains within itself a predisposition to react upon certain mechanical 
stimuli, by growth in a certain direction and to a certain extent. 
The predisposition towards an ' Exercierknochen ' is therefore already 
present, or else the growth could not be formed ; and the same 
reasoning applies to all other ' acquired characters.' 

Nothing* can arise in an organism unless the predisposition to it 

is pre-existent, for every acquired character is simply the reaction 

of the organism upon a certain stimulus. Hence I should never 

have thought of asserting that predispositions cannot be trans- 

* E. Roth, ' Die Thatsachen der Vererbung.' 2. Aufl., Berlin, 1885, p. 14. 



170 THE CONTINUITY OF THE GERM-PLASM AS THE 

mitted, as E. Roth appears to believe. For instance, I freely admit 
that the predisposition to an ' Exercierknochen ' varies, and that a 
strongly marked predisposition may be transmitted from father to 
son, in the form of bony tissue with a more susceptible constitution. 
But I should deny that the son could develope an ' Exercierknochen ' 
without having drilled, or that, after having drilled, he could develope 
it more easily than his father, on account of the drilling through 
which the latter first acquired it. I believe that this is as im- 
possible as that the leaf of an oak should produce a gall, without 
having been pierced by a gall-producing insect, as a result of the 
thousands of antecedent generations of oaks which have been pierced 
by such insects, and have thus * acquired ' the power of producing 
galls. I am also far from asserting that the germ-plasm — which, as 
I hold, is transmitted as the basis of heredity from one generation to 
another — is absolutely unchangeable or totally uninfluenced by 
forces residing in the organism within which it is transformed 
into germ-cells. I am also compelled to admit that it is conceiv- 
able that organisms may exert a modifying influence upon their 
germ-cells, and even that such a process is to a certain extent in- 
evitable. The nutrition and growth of the individual must exercise 
some influence upon its germ-cells ; but in the first place this in- 
fluence must be extremely slight, and in the second place it cannot 
act in the manner in which it is usually assumed that it takes place. 
A change of growth at the periphery of an organism, as in the case 
of an ' Exercierknochen,' can never cause such a change in the mole- 
cular structure of the germ -plasm as would augment the predis- 
position to an ' Exercierknochen,' so that the son would inherit an 
increased susceptibility of the bony tissue or even of the particular 
bone in question. But any change produced will result from the 
reaction of the germ-cell upon changes of nutrition caused by 
alteration in growth at the periphery, leading to some change 
'in the size, number, or arrangement of its molecular units. In the 
present state of our knowledge there is reason for doubting whether 
such reaction can occur at all ; but, if it can take place, ao all events 
the quality of the change in the germ-plasm can have nothing to 
do with the quality of the acquired character, but only with the 
) way in which the general nutrition is influenced by the latter. In 
^ the case of the 'Exercierknochen' there would be practically no 
change in the general nutrition, but if such a bony growth could 



FOUNDATION OF A THEORY OF HEREDITY. 171 

reach the size of a carcinoma, it is conceivable that a disturbance of 
the general nutrition of the body might ensue. Certain experi- 
ments on plants — in which Nag-eli showed that they can be sub- 
mitted to strongly varied conditions of nutrition for several genera- 
tions, without the production of any visible hereditary change — 
show that the influence of nutrition upon the germ-cells must be 
very slight, and that it may possibly leave the molecular structure 
of the germ-plasm altogether untouched. This conclusion is also 
supported by comparing the uncertainty of these results with the 
remarkable precision with which heredity acts in the case of those 
characters which are known to be transmitted. In fact, up to the 
present time, it has never been proved that any changes in general 
nutrition can modify the molecular structure of the germ-plasm, 
and far less has it been rendered by any means probable that 
the germ-cells can be aflected by acquired changes which have no 
influence on general nutrition. If we consider that each so-called 
predisposition (that is, a power of reacting upon a certain stimulus 
in a certain way, possessed by any organism or by one of its 
parts) must be innate, and further that each acquired character is 
only the predisposed reaction of some part of an organism upon 
some external influence ; then we must admit that only one of the 
causes which produce any acquired character can be transmitted, 
the one which was present before the character itself appeared, viz. 
the predisposition ; and we must further admit that the latter 
arises from the germ, and that it is quite immaterial to the follow- 
ing generation whether such predisposition comes into operation or 
not. The continuity of the germ -plasm is amply sufiicient to 
account for such a phenomenon, and I do not believe that any 
objection to my hypothesis, founded upon the actually observed 
phenomena of heredity, will be found to hold. If it be accepted, 
many facts will appear in a light difterent from that which has been 
cast upon them by the hypothesis which has been hitherto received, 
— a hypothesis which assumes that the organism produces germ- 
cells afresh, again and again, and that it produces them entirely 
from its own substance. Under the former theory the germ-cells 
are no longer looked upon as the product of the parent's body, at 
least as far as their essential part — the specific germ -plasm — is 
concerned : they are rather considered as something which is to be 
placed in contrast with the tout ensemble of the cells which make 



172 THE CONTINUITY OF THE GERM-PLASM AS THE 

up the parent's body, and the germ-cells of succeeding generations 
stand in a similar relation to one another as a series of generations 
of unicellular organisms, arising by a continued process of cell- 
division. It is true that in most cases the generations of germ-cells 
do not arise immediately from one another as complete cells, but 
only as minute particles of germ-plasm. This latter substance, 
however, forms the foundation of the germ-cells of the next genera- 
tion, and stamps them with their specific character. Previous to 
the publication of my theory, G. Jager ^, and later M. Nussbaum ^, 
have expressed ideas upon heredity which come very near to my 
own^. Both of these writers started with the hypothesis that there 

' Jager, ' Lehrbuch der allgemeinen Zoologie,' Bd. II. Leipzig, 1878. 

" M. Nussbaum, * Die DifFerenzirung des Geschlechts im Thierreich,' Arch. f. 
Mikrosk. Anat., Bd. XVIII. 1880. 

^ I have since learnt that Professor Eauber of Dorpat also expressed similar 
views in 1 880 ; and Professor Herdman of Liverpool informs me that Mr. Francis 
Galton had brought forward in 1876 a theory of heredity of which the fundamental 
idea in some ways approached that of the continuity of the germ-plasm (' Journal 
of the Anthropological Institute,' vol. v ; London, 1876). — A. W., 1888. 

[A less complete theory was brought forward by Galton at an earlier date, in 
1872 (see Proc. Roy. Soc. No. 136, p. 394). In this paper he proposed the idea that 
heredity chiefly depends upon the development of the offspring from elements directly 
derived from the fertilized ovum which had produced the parent. Galton speaks of 
the fact that ' each individual may properly be conceived as consisting of two parts, 
one of which is latent and only known to us by its effects on his posterity, while the 
other is patent, and constitutes the person manifest to our senses. The adjacent and, 
in a broad sense, separate lines of growth in which the patent and latent elements 
are situated, diverge from a common group and converge to a common contribution, 
because they were both evolved out of elements contained in a structureless ovum, 
and they, jointly, contribute the elements which form the structureless ova of their 
offspring.' The following diagram shows clearly 'that the span of each of the links in 
the general chain of heredity extends from one structureless stage to another, and 
not from person to person : — 

Structureless elements J ... Adult Father ... ) structureless elements 
in Father ( ... Latent in Father . . . i in Offspring.' 

Again Galton states — ' Out of the structureless ovum the embryonic elements are 
taken . . . and these are developed (a) into the visible adult individual ; on the 
other hand . . ., after the embryonic elements have been segregated, the large 
residue is developed (i) into the latent elements contained in the adult individual.' 
The above quoted sentences and diagram indicate that Galton does not derive the 
■whole of the hereditary tendencies from the latent elements, but that he believes 
some effect is also produced by the patent elements. When however he contrasts 
the relative power of these two influences, he attaches comparatively little importance 
to the patent elements. Thus if any character be fixed upon, Galton states that it 
'may be conceived (1) as purely personal, without the concurrence of any latent 
equivalents, (2) as personal but conjoined with latent equivalents, and (3) as existent 
wholly in a latent form.' He argues that the hereditary power in the first case is 



FOUNDATION OF A THEORY OF HEREDITY. 173 

must be a direct connexion between the germ-cells of succeeding 
generations, and they tried to establish such a continuity by sup- 
posing that the germ-cells of the offspring are separated from the 
parent germ-cell before the beginning of embryonic development, 
or at least before any histological differentiation has taken place. 
In this form their suggestion cannot be maintained, for it is in 
conflict with numerous facts. A continuity of the germ-cells does 
not now take place, except in very rare instances ; but this fact 
does not prevent us from adopting a theory of the continuity of 
the gevm-plasm, in favour of which much weighty evidence can be 
brought forward. In the following pages I shall attempt to develope 
further the theory of which I have just given a short account, to 
defend it against any objections which have been brought forward, 
and to draw from it new conclusions which may perhaps enable us 
more thoroughly to appreciate facts which are known, but im- 
perfectly understood. It seems to me that this theory of the con- 
tinuity of the germ-plasm deserves at least to be examined in all its 
details, for it is the simplest theory upon the subject, and the one 
which is most obviously suggested by the facts of the case, and we 
shall not be justified in forsaking it for a more complex theory 
until proof that it can be no longer maintained is forthcoming. 
It does not presuppose anything except facts which can be observed 
at any moment, although they may not be understood, — such as 
assimilation, or the development of like organisms from like germs ; 
while every other theory of heredity is founded on hypotheses which 
cannot be proved. It is nevertheless possible that continuity of 
the germ-plasm does not exist in the manner in which I imagine 
that it takes place, for no one can at present decide whether all the 

exceedingly feeble, because ' the effects of the use and disuse of limbs, and tliose of 
habit, are transmitted to posterity in only a very slight degree.' He also argues that 
many instances of the supposed transmission of personal characters are really due 
to latent equivalents. 'The personal manifestation is, on the average, though it 
need not be so in every case, a. certain proof of the existence of latent elements.' 
Having argued that the strength of the latter in heredity is further supported by 
the facts of reversion, Galton considers it is safe to conclude 'that the contribution 
from the patent elements is very much less than from the latent ones.' In the 
later development of his theory, Galton adheres to the conception of ' gemmules ' 
and accepts Darwin's views, although ' with considerable modification.' Together 
with pangenesis itself, Gal ton's theory must be looked upon as preformational, and 
so far it is in opposition to Weismann's theory which is epigenetic. See Appendix 
IV. to the next Essay (V.), pp. 316-319. — E. B. P.] 



174 THE CONTINUITY OF THE GERM-PLASM AS THE 

ascertained facts agree with and can be explained by it. Moreover 
the ceaseless activity of research brings to light new fects every 
day, and I am far from maintaining that my theory may not be 
disproved by some of these. But even if it should have to be 
abandoned at a later period, it seems to me that, at the present time, 
it is a necessary stage in the advancement of our knowledge, and 
one which must be brought forward and passed through, whether 
it prove right or wrong, in the future. In this spirit I offer the 
following considerations, and it is in this spirit that I should wish 
them to be received. 

L The Germ-plasm. 

I must first define precisely the exact meaning of the tenni 
germ-plasm. 

In my previous writings in which the subject has been alluded 
to, I have simply spoken of germ-plasm without indicating more 
precisely the part of the cell in which we may expect to find this 
substance — the bearer of the characteristic nature of the species 
and of the individual. In the first place such a course was sufficient 
for my immediate purpose, and in the second place the number of 
ascertained facts appeared to be insufficient to justify a more exact 
definition. I imagined that the germ-plasm was that part of a 
germ-cell of which the chemical and physical properties — including 
tbe molecular structure — enable the cell to become, under appro- 
priate conditions, a new individual of the same species. I therefore 
believed it to be some such substance as Niigeli ^, shortly afterwards, 
called idioplasm, and of which he attempted, in an admirable 
manner, to give us a clear understanding. Even at that time 
one might have ventured to suggest that the organized substance 
of the nucleus is in all probability the bearer of the phenomena of 
heredity, but it was impossible to speak upon this point with any 
degree of certainty. O. Hertwig^ and Fol^ had shown that the 
process of fertilization is attended by a conjugation of nuclei, and 
Hertwig had even then distinctly said that fertilization generally 

• Nageli, * Mechanisch-physiologiache Theorie der Abstammungslelire.' Munchen 
u. Leipzig, 1884. 

' O. Hertwig, ' Beitrage zur Kenntniss der Bildung, Befruchtung und Theilung 
des thierischen Eies.' Leipzig, 1876. 

^ Fol, ' Recherches sur la fecondation, etc' Genfeve, 1879. 

I 



FOUNDATION OP A THEORY OF HEREDITY. 175 

depends upon the fusion of two nuclei ; but the possibility of the 
co-operation of the substance of the two germ-cells could not be 
excluded, for in all the observed cases the sperm-cell was very small 
and had the form of a spermatozoon, so that the amount of its cell- 
body, if there is any, coalescing with the female cell, could not be 
distinctly seen, nor was it possible to determine the manner in 
which this coalescence took place. Furthermore, it was for some 
time very doubtful whether the spermatozoon really contained 
true nuclear substance, and even in 1879 Fol was forced to the con- 
clusion that these bodies consist of cell-substance alone. In the 
following year my account of the sperm-cells of Daphnidae followed, 
and this should have removed every doubt as to the cellular nature 
of the sperm-cells and as to their possession of an entirely normal 
nucleus, if only the authorities upon the subject had paid more 
attention to these statements^. In the same year (1880) Balfour 
summed up the facts in the following manner — ' The act of impreg- 
nation may be described as the fusion of the ovum and spermatozoon, 
and the most important feature in this act appears to be the fusion 
of a male and female nucleus^.' It is true that Calberla had already 
observed in Petromyzon, that the tail of the spermatozoon does not 
penetrate into the Q^^, but remains in the micropyle ; but on the 
other hand the head and part of the ' middle-piece ' which effect 
fertilization, certainly contain a small fraction of the cell-body in 
addition to the nuclear substance, and although the amount of the 
former which thus enters the q^^ must be very small, it might never- 
theless be amply sufficient to transmit the tendencies of heredity. 
Nageli and Pfliiger rightly asserted, at a later date, that the 
amount of the substance which forms the basis of heredity is neces- 
sarily very small, for the fact that hereditary tendencies are as 
strong on the paternal as on the maternal side, forces us to assume 
that the aijiount of this substance is nearly equal in both male 
and female germ-cells. Although I had not published anything 
upon the point, I was myself inclined to ascribe considerable 

^ KoUiker formerly stated, and has again repeated in his most recent publication, 
that the spermatozoa (' Samenfaden ') are mere nuclei. At the same time he re- 
cognizes the existence of sperm-cells in certain species. But proofs of the former 
assertion ought to be much stronger in order to be sufficient to support so improbable 
a hypothesis as that the elements of fertilization may possess a varying morpho- 
logical value. Compare Zeitschr. f. wiss. Zool., Bd. XLII. 

' F. M. Balfour, * Comparative Embryology,' vol. i. p. 69. 



176 THE CONTINUITY OP THE GERM-PLASM AS THE 

importance to the cell-substance in the process of fertilization ; 
and I had been especially led to adopt this view because my 
investigations upon Baphnidae had shown that an animal produces 
large sperm-cells with an immense cell-body whenever the economy 
of its organism permits. All Baphnidae in which internal fertiliza- 
tion takes place (in which the sperm-cells are directly discharged 
upon the unfertilized egg), produce a small number of such large 
sperm-cells (Sida, Polyphemus, Bythotrephes) ; while all species 
with external fertilization {Baphnidae, Lynceinae) produce very 
small sperm-cells in enormous numbers, thus making up for the 
immense chances against any single cell being able to reach an 
egg. Hence the smaller the chances of any single sperm-cell 
being successful, the larger is the number of such cells produced, 
and a direct result of this increase in number is a diminiition 
in size. But why should the sperm-cells remain or become so 
large in the species in which fertilization is internal? The idea 
suggests itself that the species in this way gains some advantage, 
which must be given up in the other cases ; although such ad- 
vantage might consist in assisting the development of the fertilized 
ovum and not in any increase of the true fertilizing substance. At 
the present time we are indeed disposed to recognize this advantage 
in still more unimportant matters, but at that time the ascertained 
facts did not justify us in the assertion that fertilization is a mere 
fusion of nuclei, and M. Nussbaum^ quite correctly expressed the 
state of our knowledge when he said that the act of fertilization 
consisted in ' the union of identical parts of two homologous cells.' 
Pfliiger's discovery of the 'isotropism' of the ovum was the 
first fact which distinctly pointed to the conclusion that the bodies 
of the germ-cells have no share in the transmission of hereditary 
tendencies. He showed that segmentation can be started in 
different parts of the body of the egg, if the latter be permanently 
removed from its natural position. This discovery constituted an 
important proof that the body of the egg consists of a uniform 
substance, and that certain parts or organs of the embryo cannot 
be potentially contained in certain parts of the egg, so that they 
can only arise from these respective parts and from no others. 
Pfluger was mistaken in the further interpretation, from which he 
concluded that the fertilized ovum has no essential relation to the 
' Arch. f. mikr. Anat., Bd. 23. p. 182, 1884. 



FOUNDATION OF A THEORY OF HEREDITY. 177 

organization of the animal subsequently formed by it, and that it 
is only the recun-ence of the same external conditions which 
causes the germ-cell to develope always in the same manner. The 
force of gravity was the first factor, whioh, as Pfliiger thought, 
determined the building up of the embryo : but he overlooked the 
fact that isotropism can only be referred to the body of the egg, 
and that besides this cell-body there is also a nucleus present, from 
which it was at least possible that regulative influences might 
emanate. Upon this point Born ^ first showed that the position of 
the nucleus is changed in eggs which are thus placed in unnatural 
conditions, and he proved that the nucleus must contain a principle 
which in the first place directs the formation of the embryo. Roux^ 
further showed that, even when the effect of gravity is compensated, 
the development is continued unchanged, and he therefore concluded 
that the fertilized egg contains within itself all the forces necessary 
for normal development. Finally, O. Hertwig ^ proved fiiom observa- 
tions on the eggs of sea-urchins, that at any rate in these animals, 
gravity has no directive influence upon segmentation, but that the 
position of the first nuclear spindle decides the direction which will 
be taken by the first divisional plane of segmentation. These 
observations were however still insufficient to prove that fertiliza- 
tion is nothing more than the fusion of nuclei ^. 

A further and more important step was taken when E. van 
Beneden^ observed the process of fertilization in Ascaris megalo- 
cephala. Like the investigations of Nussbaum ^ upon the same sub- 
ject, published at a rather earlier date, van Beneden's observations 
did not altogether- exclude the possibility of the participation of the 
body of the sperm-cell in the real process of fertilization ; still the 
fact that the nuclei of the egg-cell and the sperm-cell do not 

^ Bom, 'Biologische Untersuchungen,' I, Arch. Mikr. Anat., Bd. XXIV. 

^ Roux, 'Beitrage zum Entwicklungsmechanismus des Embryo,' 1884. 

' O. Hertwig, ' Welchen Einfluss iibt die Schwerkraft/ etc. Jena, 1884. 

* [Our present knowledge of the development of vegetable ova (including the 
position of the parts of the embryo) is also in favour of the view that it is not in- 
fluenced by external causes, such as gravitation and light. It takes place in a 
manner characteristic of the genus or species, and essentially depends on other causes 
which are fixed by heredity, see Heinricher ' Beeinflusst das Licht die Organanlage 
am Farnembryo ? ' in Mittheilungen aus dem Botanischen Institute zu Graz, II. 
Jena, 1888.— S. S.] 

" E. van Beneden, ' Recherches sur la maturation de I'ceuf,' etc., 1883. 

® M. Nussbaum, 'Ueber die Veranderung der Geschlechtsprodukte bia zur Ei- 
fnrchung,' Arch. Mikr. Anat., 1884. 

N 



178 THE CONTINUITY OP THE GERM-PLASM AS THE 

coalesce irregularly, but that their loops are placed reg^arly opposite 
one another in pairs and thus form one new nucleus (the first seg- 
mentation nucleus), distinctly pointed to the conclusion that the 
nuclear substance is the sole bearer of hereditary tendencies — ^that 
in fact fertilization depends upon the coalescence of nuclei. Van 
Beneden himself did not indeed arrive at these conclusions : he was 
prepossessed with the idea that fertilization depends upon the union 
of two sexually differentiated nuclei, or rather half-nuclei — ^the male 
and female pronuclei. He considered that only in this way could a 
single complete nucleus be formed, a nucleus which must of course 
be hermaphrodite, and he believed that the essential cause of further 
development lies in the fact that, at each successive division of 
nuclei and cells, this hermaphrodite nature of the nucleus is main- 
tained by the longitudinal division of the loops of each mother- 
nucleus, causing a uniform distribution of the male and female 
loops in both daughter-nuclei. 

But van Beneden undoubtedly deserves great credit for having 
constructed the foundation upon which a scientific theory of heredity 
could be built. It was only necessary to replace the terms male 
and female pronuclei, by the terms nuclear substance of the male 
and female parents, in order to gain a starting-point from which 
further advance became possible. This step was taken by Stras- 
burger, who at the same time brought forward an instance in 
which the nucleus only of the male genn-cell (to the exclusion of 
its cell-body) reaches the egg-cell. He succeeded in explaining 
the process of fertilization in Phanerogams, which had been for a 
long time involved in obscurity, for he proved that the nucleus of 
the sperm-cell (the pollen-tube) enters the embryo-sac and fuses 
with the nucleus of the egg-cell : at the same time he came to 
the conclusion that the body of the sperm-cell does not pass into 
the embryo-sac, so that in this case fertilization can only depend 
upon the fusion of nuclei ^. 

' Eduard Strasburger, ' Neue Untersuchungen tiber den Befruchtungsvorgang bei 
den Phanerogamen als Grundlage ftlr eine Theorie der Zeugung.' Jena, 1884. 

[It is now generally admitted that, in the Vascular Cryptogams, as also in Mosses 
and Liverworts, the bodies of the spemiatozoids are formed by the nuclei of the cells 
from which they arise. Only the cilia which they possess, and which obviously merely 
serve as locomotive organs, are said to arise from the surrounding cytoplasm. It is 
therefore in these plants also the nucleus of the male cell which effects the fertilization 
of the ovum. See Gobel, 'Outlines of Classification and Special Morphology,' trans- 



FOUNDATION OF A THEORY OF HEEEDITY. 179 

Thus the nuclear substance must be the sole bearer of hereditary 
tendencies, and the facts ascertained by van Beneden in the case of 
Ascaris plainly show that the nuclear substance must not only 
contain the tendencies of growth of the parents, but also those of a 
very large number of ancestors. Each of the two nuclei which 
unite in fertilization must contain the germ-nucleoplasm of both 
parents, and this latter nucleoplasm once contained and still contains 
the germ-nucleoplasm of the grandparents as well as that of all 
previous generations. It is obvious that the nucleoplasm of each 
antecedent generation must be represented in any germ-nucleus in 
an amount which becomes less as the number of intervening genera- 
tions becomes greater ; and the proportion can be calculated after 
the manner in which breeders, when crossing races, determine the 
proportion of pure blood which is contained in any of the descend- 
ants. Thus while the germ-plasm of the father or mother constitutes 
half the nucleus of any fertilized ovum, that of a grandparent only 
forms a quarter, and that of the tenth generation backwards only 
ishi) ^^^ so on. The latter can, nevertheless, exercise influence'' 
over the development of the offspring, for the phenomena of atavism 
show that the germ-plasm of very remote ancestors can occasionally 
make itself felt, in the sudden reappearance of long-lost characters. 
Although we are unable to give a detailed account of the way in 
which atavism happens, and of the circumstances under which it 
takes place, we are at least able to understand how it becomes 
possible ; for even a very minute trace of a specific germ-plasm 
possesses the definite tendency to build up a certain organism, and 
will develope this tendency as soon as its nutrition is, for some 
reason, favoured above that of the other kinds of germ-plasm present 
in the nucleus. Under these circumstances it will increase more 
rapidly than the other kinds, and it is readily conceivable that a 
preponderance in the quantity of one kind of nucleoplasm may 
determine its influence upon the cell-body. 

Strasburger — supported by van Beneden's observations, but in 
opposition to the opinions of the latter — had already explained, in 
a manner similar to that described above, the process by which the 
hereditary transmission of certain characters takes place, and to this 

lated by H. E. F. Garnsey, edited by I. B. Balfour, Oxford, 1887, p. 203, and 
Douglas H. Camiibell, 'Zur Entwicklungsgescbichte derSpermatozoiden,'in Berichte 
d. deutschen bot. Gesellschaft, vol. v (1887), p. 120. — S. S.] 

N 3 



180 THE CONTINUITY OF THE GERM-PLASM AS THE 

extent our opinions coincide. The nature of heredity is based upon 
the transmission of nuclear substance with a specific molecular con- 
stitution. This substance is the specific nucleoplasm of the germ- 
cell, to which I have given the name of germ-plasm. 

O. Hertwig^ has also come to the same conclusion : at an earlier 
date he had looked upon the coalescence of nuclei as the most 
essential feature in the process of fertilization. He now believes 
that this former opinion has been confirmed by the recent dis- 
coveries which have been shortly described above. 

Although I entirely agree with Hertwig, as far as the main 
question is concerned, I cannot share his opinions when he identi- 
fies Nageli's idioplasm with the nucleoplasm of the germ-cell. 
Nageli's idioplasm certainly includes the germ-plasm, if I may 
retain this expression for the sake of brevity. Niigeli in forming 
his hypothesis did indeed start with the germ-cells, but his idio- 
plasm not only represents the nucleoplasm of the germ-cells, but 
also that of all the other cells of the organism ; all these nucleo- 
plasms taken together constitute Nageli's idioplasm. According 
to Nageli, the idioplasm forms a network which extends through 
the whole body, and represents the specific molecular basis which 
determines its nature. Although this latter suggestion — the 
general part of his theory — is certainly valid, and although it is 
of great importance to have originated the idea of idioplasm in this 
general sense, in contrast to the somato-plasm ('Nahrplasma'), it is 
nevertheless true that we are not justified in retaining the details 
of his theory. 

In the first place the idioplasm does not form a directly con- 
tinuous network throughout the entire body ; and, secondly, the 
whole organism is not penetrated by a single substance of homo- 
geneous constitution, but each special kind of cell must contain 
the specific idioplasm or nucleoplasm which determines its nature. 
There are therefore in each organism a multitude of diflerent 
kinds of idioplasm. Thus we should be quite justified in genei-ally 
speaking of Nageli's idioplasm as nucleoplasm, and vice versa. 

It is perfectly certain that the idioplasm cannot form a con- 
tinuous network through the whole organism, if it is seated in the 
nucleus and not in the cell-body. Even if the bodies of cells are 

' O. Hertwig, ' Das Problem der Befruchtang und der Isotropie des Eies.' Jena, 
1885. 



FOUNDATION OF A THEORY OF HEREDITY. 181 

everywhere connected by fine processes (as has been proved in animals 
by Leydig- and Heitzmann, and in plants by various botanists), 
they do not form a network of idioplasm but of somato-plasm ; a 
substance which, according* to Nag-eli, stands in marked contrast to 
idioplasm. Strasburg-er has indeed already spoken of a ' cyto-idio- 
plasm,' and it is certainly obvious that the cell-body often possesses 
a specific character, but we must in all cases assume that such a 
character is impressed upon it by the influence of the nucleus, or, 
in other words, that the direction in which the cell-substance is 
differentiated in the course of development is determined by the 
quality of its nuclear substance. So far, therefore, the deter- 
mining- nuclear substance corresponds to the idioplasm alone, 
while the substance of the cell-body must be identified with the 
somato-plasm (' Nahrplasma') of Nageli. At all events, in practice, 
it will be well to restrict the term idioplasm to the regulative 
nuclear substance alone, if we desire to retain the well-chosen terms 
of Nageli' s theory. 

But the second part of Nageli's theory of the idioplasm is also 
untenable. It is impossible that this substance can have the same 
constitution everywhere in the organism and during every stage 
of its ontogeny. If this were so, how could the idioplasm effect 
the great differences whicH obtain in the formation of the various 
parts of the organism? In some passages of his work Nageli 
seems to express the same opinion; e.g. on page 31 he says, 'It 
would be i^racticable to regard — although only in a metaphorical 
sense- — the idioplasms of the different cells of an individual as them- 
selves different, inasmuch as they possess specific powers of pro- 
duction : we should thus include among these idioplasms all the con- 
ditions of the organism which bring about the display of specific ac- 
tivity on the part of cells.' It can be clearly seen from the passages 
immediately preceding and succeeding the above-quoted sentence, 
that Nageli, in speaking of these changes in the idioplasm, does 
not refer to material, but only to dynamical changes. On page ^;^ 
he lays special stress upon the statement that ' the idioplasm 
during its growth retains its specific constitution everywhere 
throughout the organism,' and it is only ' within these fixed 
stnictural limits that it changes its conditions of tension and move- 
ment, and thus alters the forms of growth and activity which are 
possible at each time and place ' Against such an interpretation 



182 THE CONTINUITY OF THE GERM-PLASM AS THE 

weighty objections can be raised. At present I will only men- 
tion that the meaning of the phrase 'conditions of tension and 
movement ' ought to be made clear, and that we ought to be 
informed how it is that mere differences in tension can produce 
as many different effects as could have been produced by differences 
of constitution. If any one were to assert that in Daphnidae, or in 
any other forms which produce two kinds of eggs, the power of de- 
veloping only after a period of rest, possessed by the winter-eggs, 
is based upon the fact that their idioplasm is identical with that 
of the summer-eggs, but is in another condition of tension, I 
should think such a hypothesis would be well worth consideration, 
for the animals which arise from the winter-eggs are identical 
with those produced in summer : the idioplasm which caused 
their formation must therefore be identical in its constitution ; and 
can only differ in the two cases, as water differs from ice. But 
the case is quite otherwise in the stages of ontogeny. How many 
different conditions of tension ought to be possessed by one and 
the same idioplasm in order to correspond to the thousand different 
structures and differentiations of cells in one of the higher organ- 
isms? In fact it would be hardly possible to form even an 
approximate conception of an explanation based upon mere ' con- 
ditions of tensions and movement.' But, furthermore, difference 
in effect should correspond, at any rate to some extent, with 
difference in cause : thus the idioplasm of a muscle-cell ought to 
differ more from that of a nerve-cell and of a digestive-cell in 
the same individual, than the idioplasm of the germ-cell of one 
individual differs from that of other individuals of the same species ; 
and yet, according to Nageli, the latter small difference in the 
effect is supposed to be due to difference of quality in the cause — 
the idioplasm, while the former fundamental difference in the his- 
tological differentiation of cells is supposed to follow from mere 
difference ' of tension and movement.' 

Nageli's hypothesis appears to be self-contradictory; for, al- 
though its author recognizes the truth of the fundamental law of 
development, and explains the stages of ontogeny as an abbre- 
viated recapitulation of phyletic stages, he nevertheless explains 
the latter by a different principle from that which he employs to 
explain the former. According to Niigeli, the stages of phylogeny 
are based upon true qualitative differences in the idioplasm : the 



FOUNDATION OF A THEORY OF HEREDITY. 183 

germ-plasm of a worm is qualitatively different from that of Am- 
phioxus, a frog", or a mammal. But if such phyletic stages occur 
crowded together in the ontogeny of a single species, they are said to 
be based upon different 'conditions of tension and movement ' of one 
and the same idioplasm ! It seems to me to be necessary to con- 
clude that if the idioplasm, in the course of phyletic development, 
undergoes any alteration in specific constitution, such alterations 
must also take place in ontogeny ; so far at least as the phyletic 
stages are repeated. Either the whole phyletic development is 
based upon different ' conditions of tension and movement,' or if 
this — as I believe — is impossible, the stages of ontogeny must 
be based upon qualitative alterations in the idioplasm. 

Involuntarily the question arises — how is it that such an acute 
thinker fails to perceive this contradiction ? But the answer is 
not far to seek, and Nageli himself indicates it when he adds these 
words to the sentence quoted above : ' It follows therefore that 
if a cell is detached as a germ-cell in any stage of ontogenetic 
development, and from any part of the organism, such a cell will 
contain all the hereditary tendencies of the parent individual.' 
In other words, if we are restricted to different 'conditions of 
tension and movement ' as an explanation, it seems to follow as a 
matter of course that the idioplasm can re-assume its original 
condition, and therefore that the idioplasm of any cell in the 
body can again become the idioplasm of the germ-cell ; for this to 
take place it is only necessary that the greater tension should 
become the less, or vice versa. But if we admit a real change 
in constitution, then the backward development of the idio- 
plasm of the cells of the body into germ-cells appears to be 
very far from a matter of course, and he who assumes it must 
bring forward weighty reasons. Nageli does not produce such 
reasons, but considers the metamorphosis of the idioplasm in on- 
togeny as mere differences in the ' conditions of tension and move- 
ment.' This phrase covers the weak part of his theory ; and I 
look upon it as a valuable proof that !C*^ageli has also felt that the 
phenomena of heredity can only find their explanation in the 
hypothesis of the continuity of the germ-plasm ; for his phrase is 
only capable of obscuring the question as to how the idioplasm 
of the cells of the body can be re-transformed into the idioplasm of 
germ-cells. 



184 THE CONTINUITY OF THE GERM-PLASM AS THE 

I am of the opinion that the idioplasm cannot be re-transformed, 
and I have defended this opinion for some years past ^, althoug'h I 
have hitherto laid especial stress on the positive aspect of the 
question, viz. on the continuity of the germ-plasm. I have 
attempted to prove that the germ-cells of an organism derive their 
essential nature from the fact that the germ-plasm of each genera- 
tion is carried over into that which succeeds it ; and I have tried to 
show that during the development of an egg into an animal, a part of 
the germ-substance — although only a minute part — passes over un- 
changed into the organism which is undergoing development, and 
that this part represents the basis from which future germ-cells 
arise. In this way it is to a certain extent possible to conceive 
how it is that the complex molecular structure of the germ-plasm 
can be retained unchanged, even in its most minute details, through 
a long series of generations. 

But how would this be possible if the germ-plasm were formed 
anew in each individual by the transformation of somatic idio- 
plasm ? And yet if we reject the ' continuity of the germ-plasm ' 
we are compelled to adopt this latter hypothesis concerning its 
origin. It is the hypothesis adopted by Strasburger, and we have 
therefore to consider how the subject presents itself from his point 
of view. 

I entirely agree with Strasburger when he says, ' The specific 
qualities of organisms are based upon nuclei ' ; and I further agree 
with him in many of his ideas as to the relation between the 
nucleus and cell-body : ' Molecular stimuli proceed from the nucleus 
into the surrounding cytoplasm ; stimuli which, on the one hand, 
control the phenomena of assimilation in the cell, and, on the 
other hand, give to the growth of the cytoplasm, which depends 
upon nutrition, a certain character peculiar to the species.' ' The 
nutritive cytoplasm assimilates, while the nucleus controls the 
assimilation, and hence the substances assimilated possess a certain 
constitution and nourish in a certain manner the cyto-idioplasm 
and the nuclear idioplasm. In this way the cytoplasm takes part 
in the phenomena of construction, upon which the specific form of 
the organism depends. This constructive activity of the cyto-idio- 
plasm depends upon the regulative influence of the nuclei.' The 

' This opinion was first expressed in my lecture, ' Ucber die Dauer des Lebens,' 
Jena, 18S2, translated as the first essay in the present volume. 



FOUNPATION OF A THEORY OF HEREDITY. 185 

nuclei therefore 'determine the specific direction in which an 
org-anism developes.' 

The opinion — derived from the recent study of the phenomena of 
fertilization — that the nucleus impresses its specific character upon 
the cell, has received conclusive and important confirmation in the 
experiments upon the regeneration of Infusoria, conducted simul- 
taneously by M. Nussbaum ^ at Bonn, and by A. Gruber ^ at 
Freiburg. Nussbaum's statement that an artificially separated 
portion of a Paramaecmm, which does not contain any nuclear 
substance, immediately dies, must not be accepted as of general 
application, for Gruber has kept similar frag-ments of other In- 
fusoria alive for several days. Moreover, Gruber had previously 
shown that individual Protozoa occur, which live in a normal 
manner, and are yet without a nucleus, althoug'h this structure is 
present in other individuals of the same species. But the meaning 
of the nucleus is made clear by the fact, published by Gruber, that 
such artificially separated fragments of Infusoria are incapable of 
regeneration, while on the other hand those fragments which con- 
tain nuclei always regenerate. It is therefore only under the in- 
fluence of the nucleus that the cell substance re-developes into the 
full type of the species. In adopting the view that the nucleus is 
the factor which determines the specific natm*e of the cell, we stand 
on a firm foundation upon which we can build with security. 

If therefore the first segmentation nucleus contains, in its mole- 
cular structure, the whole of the inherited tendencies of develop- 
ment, it must follow that during segmentation and subsequent 
cell-division, the nucleoplasm will enter upon definite and varied 
changes which must cause the differences appearing in the cells 
which are produced ; for identical cell-bodies depend, ceteris paribus, 
upon identical nucleoplasm, and conversely different cells depend 
upon differences in the nucleoplasm. The fact that the embryo 
grows more strongly in one direction than in another, that its cell- 
layers are of different nature and are ultimately differentiated 
into various organs and tissues, — forces us to accept the conclu- 
sion that the nuclear substance has also been changed in nature, 
and that such changes take place during ontogenetic development 

• M. Nussbaum, ' Sitzungber. der Niederrheinischen Gesellschaft fiir Natur- und 
Heilkunde.' Dec. 15, 1884. 

^ A. Gruber, ' Biologisches Centralblatt,' Ed. IV. No. 23, and V. No. 5. 



186 THE CONTINUITY OF THE GERM-PLASM AS THE 

in a regular and definite manner. This view is also held by Stras- 
burger, and it must be the opinion of all who seek to derive the 
development of inherited tendencies from the molecular structure 
of the germ-plasm, instead of from preformed gemmules. 

We are thus led to the important question as to the forces by 
which the determining substance or nucleoplasm is changed, and 
as to the manner in which it changes during the course of onto- 
geny, and on the answer to this question our further conclusions 
must depend. The simplest hypothesis would be to suppose that, 
at each division of the nucleus, its specific substance divides into 
two halves of unequal quality, so that the cell-bodies would also 
be transformed ; for we have seen that the character of a cell is 
determined by that of its nucleus. Thus in any Metazoon the 
first two segmentation spheres would be transformed in such a 
manner that one only contained the hereditary tendencies of the 
endoderm and the other those of the ectoderm, and therefore, at a 
later stage, the cells of the endoderm would arise from the one and 
those of the ectoderm from the other ; and this is actually known 
to occur. In the course of further division the nucleoplasm of the 
first ectoderm cell would again divide unequally, e. g. into the 
nucleoplasm containing the hereditary tendencies of the nervous 
system, and into that containing the tendencies of the external 
skin. But even then, the end of the unequal division of nuclei 
would not have been nearly reached ; for, in the formation of the 
nervous system, the nuclear substance which contains the hereditary 
tendencies of the sense-organs, would, in the course of further cell- 
division, be separated from that which contains the tendencies of 
the central organs, and the same process would continue in the 
formation of all single organs, and in the final development of the 
most minute histological elements. This process would take place 
in a definitely ordered course, exactly as it has taken place through- 
out a very long series of ancestors ; and the determining and 
directing ■ factor is simply and solely the nuclear substance, the 
nucleoplasm, which possesses such a molecular structure in the germ- 
cell that all such succeeding stages of its molecular structure in 
future nuclei must necessarily arise from it, as soon as the re- 
quisite external conditions are present. This is almost the same 
conception of ontogenetic development as that which has been held 
by embiyologists who have not accepted the doctrine of evolution : 



FOUNDATION OF A THEORY OF HEREDITY. 187 

for we have only to transfer the primary cause of development, from 
an unknown source within the organism, into the nuclear sub- 
stance, in order to make the views identical. 

It appears at first sight that the knowledge which has been 
gained by studying the indirect division of nuclei is opposed to 
such a view, for we know that each mother-loop of the so-called 
nuclear plate divides longitudinally into two exactly equal halves, 
which can be stained and thus rendered visible. 

In this way each resulting daughter-nucleus receives an equal 
supply of halves, and it therefore appears that the two nuclei must 
be completely identical. This at least is Strasburger's conclusion, 
and he regards such identity as a fundamental fact, which cannot 
be shaken, and with which all attempts at further explanation must 
be brought into accord. 

How then can the gradual transformation of the nuclear substance 
be brought about ? For such a transformation must necessarily 
take place if the nuclear substance is really the determining factor 
in development. Strasburger attempts to support his hypothesis 
by assuming that the inequality of the daughter-nuclei arises from 
unequal nutrition ; and he therefore considers that the inequality 
is brought about after the division of the nucleus and of the cell. 
Strasburger has shown, in a manner which is above all criticism, 
that the nucleus derives its nutrition from the cell-body, but then 
the cell-bodies of the two ex hypothec identical daughter-nuclei 
must be different from the first, if they are to influence their nuclei 
in diflferent ways. But if the nucleus determines the nature of 
the cell, it follows that two identical daughter-nuclei which have 
arisen by division within one mother-cell cannot come to possess 
unequal cell-bodies. As a matter of fact, however, the cell-bodies of 
two daughter-cells often differ in size, in appearance, and in their 
subsequent history, and these facts are sufficient to prove that in 
such cases the division of the nucleus must have been unequal. 
It appears to me to be a necessary conclusion that, in such an in- 
stance, the mother-nucleus must have been capable of splitting into 
nuclear substances of differing quality. I think that, in his argu- 
ment, Strasburger has over-estimated the support afforded by exact 
observations upon indirect nuclear division. Certainly the fact, 
discovered by Flemming, and more exactly studied by Balbiani and 
Pfitzner, that, in nuclear division, the loops split longitudinally, is 



188 THE CONTINUITY OF THE GERM-PLASM AS THE 

of great and even of fundamental importance. Furthermore, the 
observations, conducted last year by van Beneden, on the process 
of fertilization in Ascaris, have given to Flemming's discovery a 
clearer and more definite meaning than could have been at first 
ascribed to it. The discovery proves, in the first place, that the 
nucleus always divides into two parts of equal quantity, and fur- 
ther that in every nuclear division, each daughter-nucleus receives 
the same amount of nuclear substance from the father as from the 
mother ; but, as it seems to me, it is very far from proving that the 
quality of the parent nucleoplasms must always be equal in the 
daughter-nuclei. It is true that the fact seems to prove this ; and 
if we remember the description of the most favourable instance 
which has been hitherto discovered, viz. the process of fertilization 
in the egg of Ascaris, as represented by van Beneden, the two 
longitudinal halves of each loop certainly impress the reader as 
being absolutely identical (compare, for instance, loc. cit. Plate XIX, 
figs. I, 4, 5)- But we must not forget that we do not see the 
molecular structure of the nucleoplasm, but something which we 
can only look upon (when we remember how complex this molecular 
structure must be) as a very rough expression of its quantity. Our 
most powerful and best lenses just enable us to make out the form 
of separate stainable granules present in a loop which is about to 
divide : they appear as spheres and immediately after division as 
hemispheres. But according to Strasburger, these granules, the so- 
called microsomata, only serve for the nutrition of the nuclear sub- 
stance proper, which lies between them unstainable, and therefore 
not distinctly visible. But even if these granules represent the true 
idioplasm, their division into two exactly equal parts would give us 
no proof of equality or inequality in their constitution : il would 
only give us an idea of their quantitative relations. We can only 
obtain proofs as to the quality of the molecular structure of the 
two halves by their effect on the bodies of the daughter-cells, and 
we know that these latter are frequently different in size and 
quality. 

This point is so important that I must illustrate it by a few more 
examples. The so-called polar bodies (to be treated more in detail 
below) which are expelled during maturation from the eggs of so 
many animals, are true cells, as was first proved by Biitschli in 
Nematodes : their formation is due to a process of undoubted cell- 



FOUNDATION OF A THEORY OF HEREDITY. 189 

division usually accompanied by a typical form of indirect nuclear 
division ^. If any one is still in doubt upon this point, after the 
observations of Fol and Hertwig-, he mig-ht easily be convinced of 
its truth by a g-lance at the figures (unfortunately too little known) 
which Trinchese ^ has published, illustrating* this process in the eggs 
of certain g-astropods. The eggs of Amphorina coerulea are in every 
way suitable for observation, being entirely translucent, and having 
large distinct nuclei which differ from the green cytoplasm in 
colour. In these eggs two polar bodies are formed one after the 
other : and each of them immediately re-divides : hence it follows 
that four polar bodies are placed at the pole of the ^^^. But how 
is it that these four cells perish, while the nucleus, remaining in 
the yolk and conjugating with the sperm-nucleus, makes use of the 
whole body of the q^^ and developes into the embryo ? Obviously 
because the nature of the polar body is different from that of the egg- 
cell. But since the nature of the cell is determined by the quality 
of the nucleus, this quality must differ from the very moment of 
nuclear division. This is proved by the fact that the supernu- 
merary spermatozoa which sometimes enter the O;^^ do not con- 
jugate with the polar bodies. According to Strasbm-ger's theory, 
the objection might be urged that the different quality of.the nuclei 
is here caused by the very different quantity of cytoplasm by which 
they are surrounded and nourished ; but on the one hand the small- 
ness of the cell-bodies which surround most polar globules must 
have some explanation, and this can only be found in the nature of 
the nucleus ; and on the other hand the quantity of the cell-body 
which surrounds the polar globules of Amphorina is, as a matter of 
fact, somewhat larger than the sphere of green cytoplasm which 
surrounds the nucleus of the &^^ ! The difference between the polar 
bodies and the egg-cell can thus only be explained on the suppo- 
sition that, in the division of the nuclear spindle, two qualitatively 
different daughter-nuclei are produced. 

There does not seem to be any objection to the view that the 

* According to the observations of Nussbaum and van Beneden, the egg of Ascaria 
departs from the ordinary type, but I think that the latter observer goes too far 
when he concludes from the form of the nuclear spindle (of which the two halves are 
inclined to each other at an angle) that we have before us a process entirely differei;© 
from that of ordinary nuclear division. j 

^ Trinchese, ' I primi moment! dell' evoluzione nei moUuschi,' Atti Acad. Ly' 
(3) vii. 1879, Roma. ^'^ 



190 THE CONTINUITY OF THE GERM-PLASM AS THE 

microsomata of the nuclear loops — assuming that these bodies 
represent the idioplasm — are capable of dividing into halves, equal 
in form and appearance, but unequal in quality. We know that 
this very process takes place in many egg-cells ; thus in the 
egg of the earth-worm the first two segmentation spheres are 
equal in size and appearance, and yet the one forms the endoderm 
and the other the ectoderm of the embryo. 

I therefore believe that we must accept the hypothesis that, 
in indirect nuclear division, the formation of unequal halves may 
take place quite as readily as the formation of equal halves, and 
that the equality or inequality of the subsequently produced 
daughter-cells must depend upon that of the nuclei. Thus dm-ing 
ontogeny a gradual transformation of the nuclear substance takes 
place, necessarily imposed upon it, according to certain laws, by its 
own nature, and such transformation is accompanied by a gradual 
change in the character of the cell-bodies. 

It is true that we cannot gain any detailed knowledge of the 
nature of these changes in the nuclear substance, but we can very 
well arrive at certain general conclusions about them. If we may 
suppose, with Nageli, that the molecular structure of the germ- 
idioplasm, or according to our terminology the germ-plasm, be- 
comes more complicated according to the greater complexity of 
the organism developed from it, then the following conclusions will 
also be accepted, — that the molecular structure of the nuclear 
substance is simpler as the differences between the structures 
arising from it become less ; that therefore the nuclear substance 
of the segmentation-cell of the earth-worm, which potentially con- 
tains the whole of the ectoderm, possesses a more complicated 
molecular structure than that of a single epidermic cell or nerve- 
cell. These conclusions will be admitted when it is remembered i 
that every detail in the whole organism must be represented in) 
the germ-plasm by its own special and peculiar arrangement of the [ 
groups of molecules (the micellae of Nageli), and that the germ- 
plasm not only contains the whole of the quantitative and qualita- 
tive characters of the species, but also all individual variations 
extis far as these are hereditary : for example the small depression 
bel(J the centre of the chin noticed in some families. The physical 
manyses of all apparently unimportant hereditary habits or struc- 
Nemas, of hereditary talents, and other mental peculiarities, must 



FOUNDATION OF A THEORY OF HEREDITY. 191 

all be contained in the minute quantity of germ-plasm which is 
possessed by the nucleus of a germ-cell ; — not indeed as the pre- 
formed germs of structure (the gemmules of pangenesis), but as 
variations in its molecular constitution ; if this be impossible, 
such characters could not be inherited. Nageli has shown in his 
work, which is so rich in suggestive ideas, that even in so minute 
a space as the thousandth of a cubic millimetre, such an enormous 
number (400,000,000) of micellae may be present, that the 
most diverse and complicated arrangements become possible. It 
therefore follows that the molecular structure of the germ-plasm in 
the germ-cells of an individual must be distinguished from that 
of another individual by certain differences, although these may 
be but small ; and it also follows that the germ- plasm of any 
species must differ from that of all other species. 

These considerations lead us to conclude that the molecular 
structure of the germ-plasm in all higher animals must be 
excessively complex, and, at the same time, that this complexityi 
must gradually diminish during ontogeny as the structures still toi 
be formed from any cell, and therefore represented in the mole- 
cular, constitution of its nucleoplasm, become less in number. I do 
not mean to imply that the nucleoplasm contains preformed struc- 
tures which are gradually reduced in number as they are given off 
in various directions during the building-up of organs : I mean 
that the complexity of the molecular structure decreases as the po- 
tentiality for further development also decreases, such potentiality 
being represented in the molecular structure of the nucleus. The 
nucleoplasm, which in the grouping of its particles contains po- 
tentially a hundred different modifications of this substance, must 
possess far more numerous kinds and far more complex arrange- 
ments of such particles than the nucleoplasm which only con- 
tains a single modification, capable of determining the character 
of a single kind of cell. The development of the nucleoplasm 
during ontogeny may be to some extent compared to an army 
composed of corps, which are made up of divisions, and these 
of brigades, and so on. The whole army may be taken to re- 
present the nucleoplasm of the germ-cell : the earliest cell-division 
(as into the first cells of the ectoderm and endoderm) may be 
represented by the separation of the two corps, similarly formed 
but with different duties : and the following cell- divisions by the 



192 THE CONTINUITY OF THE GERM-PLASM AS THE 

successive detachment of divisions, brigades, regiments, battalions, 
companies, etc. ; and as the groups become simpler so does their 
sphere of action become limited. It must be admitted that this 
metaphor is imperfect in two respects, first, because the quantity 
of the nucleoplasm is not diminished, but only its complexity, and 
secondly, because the strength of an army chiefly depends upon its 
numbers, not on the complexity of its constitution. And we must 
also guard against the supposition that unequal nuclear division 
simply means a separation of part of the molecular structure, 
like the detachment of a regiment from a brigade. On the con- 
trary, the molecular constitution of the mother-nucleus is certainly 
changed during division in such a way that one or both halves 
receive a new structure which did not exist before their forma- 
tion. 

My opinion as to the behaviour of the idioplasm during 
ontogeny, not only differs from that of Nageli, in that the latter 
maintains that the idioplasm only undergoes changes in its ' con- 
ditions of tension and movement,' but also because he imagines 
this substance to be composed of the preformed germs of structures 
('Anlagen'). Nageli's views are obviously bound up with his 
theory of a continuous network of idioplasm throughout the whole 
body; perhaps he would have adopted other conclusions had he 
been aware of the fact that the idioplasm must only be sought for 
in the nuclei. Nageli's views as to ontogeny can be best seen in 
the following passages : ' As soon as ontogenetic development 
begins, the groups of micellae in the idioplasm which effect the first 
stage of development, enter upon active growth : such activity 
causes a passive growth ''of the other groups, and an increase in 
the whole idioplasm, perhaps to many times its former bulk. But 
the intensities of growth in the two series of groups are unequal, 
and consequently an increasing tension is produced which sooner 
CY later, according to the number, arrangement, and energy of the 
active groups, necessarily renders the continuation of the process 
impossible. In consequence of such disturbance to the equilibrium, 
active growth now takes place in the next group, leading to fresh 
irritation, and this group then reacts more strongly than all the 
others upon the tension which first stimulated its activity. These 
changes are repeated until all the groups are gone through, and 
the ontogenetic development finally reaches the stage at which 



FOUNDATION OF A THEORY OF HEREDITY. 1^3 

propag-ation takes place, and thus the original stage of the germ is 
reached.' 

Hence, according to Nageli, the different stages of ontogeny arise 
out of the activities of different parts of the idioplasm : certain 
groups of micellae in the idioplasm represent the germs (' Anlagen ') 
of certain structures in the organism : when any such germ reacts 
under stimulation it produces the corresponding structure. It seems 
to me that this hypothesis bears some resemblance to Darwin's 
theory of pangenesis. I think that Nageli's preformed germs of 
structures (' Anlagen ') and his groups of such germs are highly 
elaborated equivalents of the gemmules of pangenesis, which, 
according to Darwin, manifest activity when their turn comes, or, 
according to Nageli, when they react under stimulation. When a 
group of such germs, by their active growth or by their ' irritation,' 
have caused a similar active growth or a similar irritation in the 
next group, the former may come to rest, or may remain in a 
state of activity together with its successor, for a longer or 
shorter period. Its activity may even last for an unlimited time, 
as is the case in the formation of leafy shoots in many plants.' 

Here, again, we recognize the fact that Nageli's whole hypothesis 
is intimately connected with the supposition that the entire mass 
of idioplasm is continuous throughout the organism. Sometimes 
one part of the idioplasm and sometimes another part is irritated, 
and then produces the corresponding organ. But if, on the other 
hand, the idioplasm does not represent a directly continuous mass, 
but is composed of thousands of single nucleoplasms which only 
act together through the medium of their cell-bodies, then we 
must substitute the conception of ' ontogenetic stages of develop- 
ment of the idioplasm ' for the conception of germs of structure 
(' Anlagen '). The different varieties of nucleoplasm which arise 
during ontogeny represent, as it were, the germs of Nageli (' An- 
lagen '), because, by means of their molecular structure, they create 
a specific constitution in the cell-bodies over which they have 
control, and also because they determine the succession of futm*e 
nuclei and cells. 

It is in this sense, and no other, that I can speak of the presence 
of preformed germs (' Anlagen ') in the idioplasm. We may sup- 
pose that the idioplasm of the first segmentation nucleus is but 
slightly different from that of the second ontogenetic stage, viz. 

o 



194 THE CONTINUITY OF THE GERM-PLASM AS THE 

that of the two following* segrnentation nuclei. Perhaps only 
a few groups of micellae have been displaced or somewhat differ- 
ently arranged. But nevertheless such groups of micellae were not 
the germs (* Anlagen ') of a second stage which pre-existed in the 
first stage, for the two are distinguished by the possession of a 
different molecular structure. This structure in the second stage, 
under normal conditions of development, again brings about the 
change by which the different molecular structure of the third 
stage is produced, and so on. 

It may be argued that von Baer's well-known and fundamental 
law of development is opposed to the hypothesis that the idioplasm 
of successive ontogenetic stages must gradually assume a simpler 
molecular structure. The organization of the species has, on the 
whole, increased immensely in complexity during the course of 
phylogeny : and if the phyletic stages are repeated in the ontogeny, 
it seems to follow that the structure of the idioplasm must 
become more complex in the course of ontogeny instead of becoming 
simpler. But the complexity of the whole organism is not repre- 
sented in the molecular structure of the idioplasm of any single 
nucleus, but by that of all the nuclei present at any one time. It 
is true that the germ-cell, or rather the idioplasm of the germ- 
nucleus, must gain greater complexity as the organism which arises 
from it becomes more complex ; but the individual nucleoplasms of 
each ontogenetic stage may become simpler, while the whole mass 
of idioplasms in the organism (which, taken together, represent the 
stage in question) does not by any means lose in complexity. 

If we must therefore assume that the molecular structure of the 
nucleoplasm becomes simpler in the course of ontogeny, as the 
number of structures which it potentially contains become smaller, 
it follows that the nucleoplasm in the cells of fully differentiated 
tissues — such as muscle, nerve, sense-organs, or glands — must 
possess relatively the most simple molecular structure ; for it cannot 
originate any fresh modification of nucleoplasm, but can only con- 
tinue to produce cells of the same structure, although it does not 
always retain this power. 

We are thus brought back to the fundamental question as to 
how the germ-cells arise in the organism. Is it possible that 
the nucleoplasm of the germ-cell, with its immensely complex 
molecular structure, potentially containing all the specific pecu- 



FOUNDATION OF A THEORY OF HEREDITY. 195 

liarities of an individual, can arise from the nucleoplasm of any 
of the body-cells, — a substance which, as we have just seen, has 
lost the power of originating* any new kind of cell, because of the 
continual simplification of its structure during development? It 
seems to me that it would be impossible for the simple nucleo- 
plasm of the somatic cells to thus suddenly acquire the power of 
originating the most complex nucleoplasm from which alone the 
entire organism can be built up : I cannot see any evidence for the 
existence of a force which could effect such a transformation. 

This difficulty has already been appreciated by other writers. 
Nussbaum's ^ theoretical views, which I have already mentioned, 
also depend upon the hypothesis that cells which have once become 
ditferentiated for the performance of special functions cannot be 
re-transformed into sexual cells : he also concludes that the latter 
are separated from all other cells at a very early period of embry- 
onic development, before any histological differentiation has taken 
place. Valaoritis ^ has also recognised that the transformation of 
histologically differentiated cells into sexual cells is impossible. 
He was led to beKeve that the sexual cells of Vertebrata arise 
from the white blood corpuscles, for he looked upon these latter 
as differentiated to the smallest extent possible. Neither of these 
views can be maintained. The former, because the sexual cells 
of all plants and most animals are not, as a matter of fact, separated 
from the somatic cells at the beginning of ontogeny; the latter, 
because it is contradicted by the fact that the sexual cells of 
vertebrates do not arise from blood corpuscles, but from the germinal 
epithelium. But even if this fact had not been ascertained we 
should be compelled to reject Valaoritis' hypothesis on theoretical 
grounds, for it is an error to assume that white blood corpuscles 
are undifferentiated, and that their nucleoplasm is similar to the 
germ-plasm. There is no nucleoplasm like that of the germ- 
cell in any of the somatic cells, and no one of these latter can be 
said to be undifferentiated. All somatic cells possess a certain 
degree of differentiation, which may be rigidly limited to one 
single direction, or may take place in one of many directions. All 
these cells are widely different from the egg-cell from which they 
originated : they are all separated from it by many generations of 

^ M. NuBsbaum, ' Archiv fiir Mikroskopische Anatomic,' Bd. XVIII und XXIII. 
'' Valaoritis, * Die Genesis des Thier-Eies.' Leipzig, 1882. 

O 3 



196 THE CONTINUITY OF THE QERM-PLASM AS THE 

cells, and this fact implies that their idioplasms possess a widely 
different structure from the idioplasm, or germ-plasm, of the egg- 
cell. Even the nuclei of the two first segmentation spheres cannot 
possess the same idioplasm as that of the first segmentation nucleus, 
and it is, of course, far less possible for such an idioplasm to be pre- 
sent in the nucleus of any of the later cells of the embryo. The 
structure of the idioplasm must necessarily become more and more 
difierent from that of the first segmentation nucleus, as the de- 
velopment of the embryo proceeds. The idioplasm of the first 
segmentation nucleus, and of this nucleus alone, is germ-plasm, and 
possesses a structure such that an entire organism can be pro- 
duced from it. Many writers appear to consider it a matter of 
course that any embryonic cell can reproduce the entire organism, 
if placed under suitable conditions. But, when we carefully look 
into the subject, we see that such powers are not even possessed by 
those cells of the embryo which are nearest to the egg-cell — viz. 
the first two segmentation spheres. We have only to remember 
the numerous cases in which one of them forms the ectoderm of 
the animal while the other produces the endoderm, in order to 
admit the validity of this objection. 

But if the first segmentation spheres are not able to develope into 
a complete organism, how can this be the case with one of the 
later embryonic cells, or one of the cells of the fully developed 
animal body ? It is true that we speak of certain cells as being 
* of embryonic character,' and only recently Kolliker ^ has given a 
list of such cells, among which he includes osteoblasts, cart.ilage 
cells, lymph corpuscles, and connective tissue corpuscles : but even 
if these cells really deserve such a designation, no explanation of the 
formation of germ-cells is afforded, for the idioplasm of the latter 
must be widely different from that of the former. 

It is an error to suppose that we gain any further insight into 
the formation of germ -cells by referring to these cells of so-called 
' embryonic character,' which are contained in the body of the 
mature organism. It is of course well known that many cells are 
characterized by very sharply defined histological differentiation, 
while others are but slightly differentiated ; but it is as difficult to 
imagine that germ-cells can arise from the latter as from the 
former. Both classes of cells contain idioplasm with a structure 

* Kolliker, ' Die Bedeutung der Zelikerne,' etc. ; Zeitschr. f. wiss. Zool. Bd. XLII. 



FOUNDATION OP A THEORY OF HEREDITY. 197 

different from that which is contained in the germ-cell, and we 
have no right to assume that any of them can form germ-cells until 
it is proved that somatic idioplasm is capable of undergoing re- 
transformation into germ-idioplasm. 

The same argument applies to the cells of the embryo itself, and 
it therefore follows that those instances of early separation of 
sexual from somatic cells, upon which I have often insisted as 
indicating the continuity of the germ-plasm, do not now appear to 
be of such conclusive importance as at the time when we were not 
sure about the localization of the idioplasm in the nuclei. In the 
great majority of cases the germ-cells are not separated at the 
beginning of embryonic development, but only in some one of the 
later stages. A single exception is found in the pole-cells (' Pol- 
zellen ') of Diptera, as was shown many years ago by Robin ^ and 
myself^. These are the first cells formed in the egg, and accord- 
ing to the later observations of Metschnikoff^ and Balbiani*, they 
become the sexual glands of the embryo. Here therefore the 
germ-plasm maintains a true unbroken continuity. The nucleus 
of the egg-cell directly gives rise to the nuclei of the pole-cells, 
and there is every reason to believe that the latter receive un- 
changed a portion of the idioplasm of the former, and with it 
the tendencies of heredity. But in all other cases the germ-cells 
arise by division from some of the later embryonic cells, and as 
these belong to a more advanced ontogenetic stage in the de- 
velopment of the idioplasm, we can only conclude that continuity 
is maintained, by assuming (as I do) that a small part of the germ- 
plasm persists unchanged during the division of the segmentation 
nucleus and remains mixed with the idioplasm of a certain series of 
cells, and that the formation of true germ-cells is brought about at 
a certain point in the series by the appearance of cells in which the 
germ-plasm becomes predominant. But if we accept this hypo- 
thesis it does not make any difference, theoretically, whether the 
germ-plasm becomes predominant in the third, tenth, hundredth, 
or millionth generation of cells. It therefore follows that cases 
of early separation of the germ-cells afford no proof of a direct 

* 'Compt. rend.' Tom. LIV. p. 150. 

^ ' Entwicklung der Dipteren,' Leipzig, 1864. 
'' 'Zeitschr. f. wiss. Zool.' Bd. XVI. p. 389 (1866). 

* 'Compt. rend.' Nov. 13, 1882. 



198 THE CONTINUITY OF THE GERM-PLASM AS THE 

persistence of the parent germ-cells in those of the offspring ; for a 
cell the offspring of which become partly somatic and partly germ- 
cells cannot itself have the characters of a germ-cell ; but it may 
nevertheless contain germ -idioplasm, and may thus transfer the sub- 
stance which forms the basis of heredity from the germ of the 
parent to that of the offspring. 

If we are unwilling to accept this hypothesis, nothing remains 
but to credit the idioplasm of each successive ontogenetic stage with 
a capability of re-transformation into the first stage. Strasburger 
accepts this view ; and he believes that the idioplasm of the nuclei 
changes during the course of ontogeny, but returns to the condition 
of the first stage of the germ, at its close. But the rule of pro- 
bability is against such a suggestion. Suppose, for instance, that 
the idioplasm of the germ-cell is characterized by ten different 
qualities, each of which may be arranged relatively to the others in 
two different ways, then the probability in favour of any given 

combination would be represented bv the fraction (-) = : 

^ ' ^2,^ 1024 

that is to say, the re-transformation of somatic idioplasm into germ- 
plasm will occur once in 1024 times, and it is therefore impossible 
for such re-transformation to become the rule. It is also obvious 
that the complex structui-e of the germ-plasm which potentially 
contains, with the likeness of a faithful portrait, the whole in- 
dividuality of the parent, cannot be represented by only ten charac- 
ters, but that there must be an immensely greater number ; it is 
also obvious that the possibilities of the arrangement of single 
characters must be assumed to be much larger than two ; so that we 

get the formula (-) , where p represents the possibilities, and n the 

characters. Thus if n and j) are but slightly larger than we 
assumed above, the probabilities become so slight as to altogether 
exclude the hypothesis of a re-transformation of somatic idioplasm 
into germ-plasm. 

It may be objected that such re-transformation is much more 
probable in the case of those germ-cells which separate early 
from the somatic cells. Nothing can in fact be urged against 
the possibility that the idioplasm of (e. g.) the third generation of 
cells may pass back into the condition of the idioplasm of the germ- 
cell ; although of course the mere possibility does not prove the 



FOUNDATION OF A THEORY OF HEREDITY. 199 

fact. But there are not many cases in which the sexual cells are 
separated so early as the third generation : and it is very rare for 
them to separate at any time during* the true segmentation of the 
egg. In Dap/midae [Mohia) separation occurs in the fifth stage of 
segmentation ^, and although this is unusually early it does not 
happen until the idioplasm has changed its molecular structure six 
times. In Sagitta ^ the separation does not take place until the 
archenteron is being formed, and this is after several hundred 
embryonic cells have been produced, and thus after the germ- 
plasm has changed its molecular structure ten or more times. But 
in most cases, separation takes place at a much later stage ; thus in 
Hydroids it does not happen until after hundreds or thousands of 
cell-generations have been passed through ; and the same fact holds 
in the higher plants, where the production of germ-cells frequently 
occurs at the end of ontogeny. In such cases the probability of a 
re-transformation of somatic idioplasm into germ-plasm becomes 
infinitely small. 

It is true that these considerations only refer to a rapid and 
sudden re-transformation of the idioplasm. If it could be proved 
that development is not merely in appearance but in reality a 
cyclical process, then nothing could be urged against the occur- 
rence of re-transformation. It has been recently maintained by 
Minot" that all development is cyclical, but this is obviously 
incorrect, for Nageli has already shown that direct non-cyclical 
courses of development exist, or at all events courses in which the 
earliest condition is not repeated at the close of development. The 
phyletic development of the whole organic world clearly illus- 
trates a development of the latter kind ; for although we may 
assume that organic development is not nearly concluded, it is 
nevertheless safe to predict that it will never revert to its original 
starting-point, by backward development over the same course 
as that which it has already traversed. No one can believe that 
existing Phanerogams will ever, in the future history of the world, 
retrace all the stages of phyletic development in precise inverse 
order, and thus return to the form of unicellular Algae or Monera ; 
or that existing placental mammals will develope into Marsupialia, 

^ Grobben, 'Arbeiten d. Wien. Zool. Instituts,' Bd. II. p. 203. 
* Butschli, ' Zeitschrift f. wiss. Zbol.' Bd. XXIII. p. 409. 
^ ' Snience,' vol. iv. No. 90, 1884. 



200 THE CONTINUITY OF THE GEBM-PLASM AS THE 

Monotremata, mammal-like reptiles, and the lower vertebrate forms, 
into worms and finally into Monera. But how can a course of 
development, which seems to be impossible in phylogeny, occur as 
the regular method of ontogeny ? And quite apart from the 
question of possibility, we have to ask for proofs of the actual 
occurrence of cyclical development. Such a proof would be af- 
forded if it could be shown that the nucleoplasm of those somatic 
cells which (e.g. in Hydroids) are transformed into germ-cells 
passes backwards through many stages of development into the 
nucleoplasm of the germ-cell. It is true that we can only recognise 
differences in the structure of the idioplasm by its effects upon the 
cell-body, but no effects are produced which indicate that such 
backward development takes place. Since the course of onward de- 
velopment is compelled to pass through the numerous stages which 
are implied in segmentation and the subsequent building-up of the 
embryo, etc., it is quite impossible to assume that backward develop- 
ment would take place suddenly. It would be at least necessary to 
suppose that the cells of embryonic character, which are said to be 
transformed into primitive germ-cells, must pass back through at any 
rate the main phases of their ontogeny. A sudden transformation 
of the nucleoplasm of a somatic cell into that of a germ-cell would 
be almost as incredible as the transformation of a mammal into an 
amoeba ; and yet we are compelled to admit that the transforma- 
tion must be sudden, for no trace of such retrogressive stages of 
development can be seen. If the appearance of the whole cell gives 
us any knowledge as to the structure of its nuclear idioplasm, we 
may be sure that the development of a primitive germ-cell proceeds 
without a break, from the moment of its first recognizable formation, 
to the ultimate production of distinct male or female sexual cells. 

I am well aware that Strasburger has stated that, in the ulti- 
mate maturation of the sexual cells, the substance of the nuclei 
returns to a condition similar to that which existed at the begin- 
ning of ontogenetic development ; still such a statement is no 
proof, but only an assumption made to support a theory. I am 
also aware that Nussbaum and others believe that, in the formation 
of spermatozoa in higher animals, a backward development sets in 
at a certain stage ; but even if this interpretation be correct, such 
backward development would only lead as far as the primitive germ- 
cell, and would afford no explanation of the further transformation 



FOUNDATION OF A THEORY OF HEREDITY. 201 

of the idioplasm of this cell into germ-plasm. But this latter 
transformation is just the point which most needs proof upon any 
theory except the one which a»3sumes that the primitive g-erm-cell 
still contains unchanged germ-plasm. Every attempt to render 
probable such a re-transformation of somatic nucleoplasm into germ- 
plasm breaks down before the facts known of the Hydroids, in 
which only certain cells in the body, out of the numerous so-called 
embryonic cells, are capable of becoming primitive germ-cells, while 
the rest do not possess this power. 

I must therefore consider as erroneous the hypothesis which 
assumes that the somatic nucleoplasm may be transformed into 
germ-plasm. Such a view may be called ' the hypothesis of the 
cyclical development of the germ-plasm.' 

Nageli has tried to support such an hypothesis on phyletic 
grounds. He believes that phyletic development follows from an 
extremely slow but steady change in the idioplasm, in the direction 
of greater complexity, and that such changes only become visible 
periodically. He believes that the passage from one phyletic stage 
to another is chiefly due to the fact that ' in any ontogeny, the 
very last structural change upon which the separation of germs 
depends, takes place in a higher stage, one or more cell-generations 
later ' than it occurred in a lower stage. ' The last structural 
change itself remains the same, while the series of structural 
changes immediately preceding it is increased.' I believe that 
Nageli, being a botanist, has been too greatly influenced by the 
phenomena of plant-life. It is certainly true that in plants, and 
especially in the higher forms, the germ-cells only make their 
appearance, as it were, at the end of ontogeny; but facts such 
as these do not hold in the animal kingdom: at any rate they 
are not true in the great majority of cases. In animals, as I have 
already mentioned several times, the germ-cells are separated from 
the somatic cells during embryonic development, sometimes even at 
its very commencement ; and it is obvious that this latter is the 
original, phyletically oldest, mode of formation. The facts at our 
disposal indicate that the germ-cells only appear, for the first time, 
after embryological development, in those cases where the forma- 
tion of asexually produced colonies takes place, either with or with- 
out alternation of generations ; or in cases where alternation of 
generations occurs without the formation of such colonies. In 



202 THE CONTINUITY OF THE GERM-PLASM AS THE 

a colony of polypes, the germ-cells are produced by the later genera- 
tions, and not by the founder of the colony which was developed 
from an egg. This is also true of the colonies of Siphonophora, 
and the germ-cells appear to arise very late in certain instances of 
protracted metamorphosis (Echinodermata), but on the other hand, 
they arise during the embryonic development of other forms (In- 
secta) which also undergo metamorphosis. It is obvious that the 
phyletic development of colonies or stocks must have succeeded 
that of single individuals, and that the formation of germ-cells in 
the latter must therefore represent the original method. Thus 
the germ-cells originally arose at the beginning of ontogeny and 
not at its close, when the somatic cells are formed. 

This statement is especially supported by the history of cer- 
tain lower plants, or at any rate chlorophyll-containing organisms, 
and I think that these forms supply an admirable illustration of 
my theory as to the phyletic origin of germ-cells, as explained in 
my earlier papers upon the same subject. 

The phyletic origin of germ-cells obviously coincides with the 
differentiation of the first multicellular organisms by division of 
labour ^. If we desire to investigate the relation between germ- 
cells and somatic cells, we must not only consider the highly 
developed and strongly differentiated multicellular organisms, but 
we must also turn our attention to those simpler forms in which 
phyletic transitions are represented. In addition to soUtaiy 
unicellular organisms, we know of others living in colonies of which 
the constituent units or cells (each of them equivalent to a uni- 
cellular organism) are morphologically and physiologically identical. 
Each unit feeds, moves, and under certain circumstances is capable 
of reproducing itself, and of thus forming a new colony by repeated 
division. The genus Pandorina (Fig. I), belonging to the natural 
order Volvocineae, represents such 'homoplastid' (Gotte) organisms. 
It forms a spherical colony composed of ciliated cells, all of which 
are exactly alike : they are embedded in a colourless gelatinous 
mass. Each cell contains chlorophyll, and possesses a red eye-spot, 
and a pulsating vacuole. These colonies are propagated by the 

* Among unicellular organisms, encysted individuals are often called germs. 
They sometimes differ from the adult organism in their smaller size and simpler 
structure (Gregarinidae), but they represent the same morphological stage of in- 
dividuality. 



FOUNDATION OP A THEORY OF HEREDITY, 



203 



sexual and asexual (Fig. II) methods alternately, although in the 
former case the conjugating swarm-cells cannot be distinguished 
with certainty as male or female. In both kinds of reproduction, 
each cell in the colony acts as a reproductive cell ; in fact, it 
behaves exactly like a unicellular organism. 




.y s ^ 



,2 * 









a 



*a> >-> 



5 '4i 



bo 

Ph m 



<i\ ^ ^ 



e8 ^ 






■-^ :3 Q 



K :=! a s 

O O' O v*^ 

e ^ - S 

1^1 " 



Rh 



O Ph 

n «8 



It is very interesting to find in another genus belonging to the 
same natural order, that the transition from the homoplastid to the 



204 THE CONTINUITY OP THE GERM-PLASM AS THE 

heteroplastid condition, and the separation into somatic and repro- 
ductive cells, have taken place. In Volvox (Fig. Ill) the spherical 
colony consists of two kinds of cells, viz. of very numerous small cili- 
ated cells, and of a much smaller number of large germ-cells without 
cilia. The latter alone possess the power of producing a new colony, 
and this takes place by the asexual and sexual methods alternately : 
in the latter a typical fertilization of large egg-cells by small sper- 
matozoa occurs. The sexual differentiation of the germ-cells is not 
material to the question we are now considering ; the important 
point is to ascertain whether here, at the very origin of heteroplastid 
organisms, the germ-cells, sexually differentiated or not, arise from 
the somatic cells at the end of ontogeny, or whether the substance of 
the parent germ-cell, during embryonic development, is from the first 
separated into somatic and germ-cells. The former interpretation 
would support Nageli's view, the latter would support my own. But 
Kirchner ^ distinctly states that the germ-cells of Volvox are differ- 
entiated during embryonic development, that is, before the escape of 
the young heteroplastid organism from the egg-capsule. We cannot 
therefore imagine that the phyletic development of the first hetero- 
plastid organism took place in a manner different from that 
which I have previously advocated on theoretical grounds, before 
this striking instance occurred to me. The germ-plasm (nucleo- 
plasm) of some homoplastid organism (similar to Pandorina) must 
have become modified in molecular structure during the course of 
phylogeny, so that the colony of cells produced by its division was 
no longer made up of identical units, but of two different kinds. 
After this separation, the germ-cells alone retained the power of 
reproduction possessed by all the parent cells, while the rest only 
retained the power of producing similar cells by division. Thus 
Volvox seems to afford distinct evidence that in the phyletic 
origin of the heteroplastid groups, somatic cells were not, as Nageli 
supposes, intercalated between the mother germ-cell and the daughter 
germ-cells in each ontogeny, but that the somatic cells arose 
directly from the former, with which they were previously identical, 
as they are even now in the case of Pandorina. Thus the con- 
tinuity of the germ-plasm is established at least for the beginning 
of the phyletic series of development. 

' ' Compare Butschli in Bronn'a ' Klassen und Ordnungen des Thierreichs,' Bd. I. 
P- 777- 



FOUNDATION OP A THEORY OF HEREDITY. 205 

The fact, already often mentioned, that in most higher* organisms 
the separation of germ-cells takes place later, and often very late, 
at the end of the whole ontogeny, proves that the time at which 
this separation of the two kinds of cells took place, must have been 
gradually changed. In this respect the well-established instances 
of early separation are of great value, because they serve to connect 
the extreme cases. It is quite impossible to maintain that the 
germ-cells of Hydroids or of the higher plants, exist from the 
time of embryonic development, as indifferent cells, which cannot 
be distinguished from others, and which are only differentiated at a 
later period. Such a view is contradicted by the simplest mathe- 
matical consideration ; for it is obvious that none of the relatively 
few cells of the embryo can be excluded from the enormous increase 
by division, which must take place in order to produce the large 
number of daughter individuals which form a colony of polypes. 
It is therefore clear that all the cells of the embryo must for a long 
time act as somatic cells, and none of them can be reserved as 
germ-cells and nothing else : this conclusion is moreover confirmed 
by direct observation. The sexual bud of a Coryne arises at a 
part of the Polype which does not in any way differ from sur- 
rounding areas, the body wall being uniformly made up of two 
single layers of cells, the one forming the ectoderm and the other the 
endoderm. Rapid growth then takes place at a single spot, and 
some of the young cells thus produced are transformed into germ- 
cells, which did not previously exist as separate cells. 

Strictly speaking I have therefore fallen into an inaccuracy in 
maintaining (in former works) that the germ-cells are themselves 
immortal ; they only contain the undying part of the organism — 
the germ-plasm ; and although this substance is, as far as we 
know, invariably surrounded by a cell-body, it does not always 
control the latter, and thus confer upon it the character of a 
germ-cell. But this admission does not materially change our 
view of the whole subject. We may still contrast the germ-cells, 
as the undying part of the Metazoan body, with the perishable 
somatic cells. If the nature and the character of a cell is deter- 
mined by the substance of the nucleus and not by the cell-body, 
then the immortality of the germ-cells is preserved, although only 
the nuclear substance passes uninterruptedly from one generation 
to another. 



206 THE CONTINUITY OF THE QERM-PLASM AS THE 

G. Jager ^ was the first to state that the body in the higher 
organisms is made up of two kinds of cells, viz., ontogenetic and 
phyletic cells, and that the latter, the reproductive cells, are 
not a product of the former (the body-cells), but that they arise 
directly from the parent germ-cell. He assumed that the formation 
of germ-cells takes place at the earliest stage of embryonic life, and 
he thus believed the connexion between the germ-plasm of the 
parent and of the offspring had received a satisfactory explana- 
tion. As I have previously mentioned in the introduction, Nuss- 
baum also brought forward this hypothesis at a later period, and 
also based it upon a continuity of the germ-cells. He assumed 
that the fertilized egg is divided into the cells of the individual and 
into the cells which effect the preservation of the species, and he 
supported this view by referring to the few known cases of early 
separation of the sexual cells. He even maintained this hypothesis 
when I had proved in my investigations on Hydromedusae that the 
sexual cells are not always separated from the somatic cells during 
embryonic development, but often at a far later period. Not t)nly 
is the hypothesis of a direct connexion between the germ-cells of 
the offspring and parent broken down by the facts known in the 
Hydroids, and in the Phanerogams^ which resemble them in this 
respect, but even the instances of early separated germ-cells quoted 
by Jager and Nussbaum do not as a matter of fact support their 
hypothesis. Among existing organisms it is extremely rare for the 
germ-cells to arise directly from the parent egg-cell (as in Diptera). 
If, however, the germ-cells are separated only a few cell-generations 
later, the postulated continuity breaks down ; for an embryonic 
cell, of which the offspring are partly germ-cells and partly somatic 
cells, cannot itself possess the nature of a germ-cell, and its idioplasm 

^ Gustav Jager, ' Lehrbuch der Allgemeinen Zoologie,' Leipzig, 1878; II. 
Abtheilung. Probably on account of the extravagant and superficial speculationa 
of the author, the valuable ideas contained in his book have been generally over- 
looked. It is only lately that I have become aware of Jiiger's above-mentioned hy- 
pothesis. M. Nussbaum seems to have also arrived at the same conclusion quite 
independently of Jager. The latter has not attempted to work out his hypothesis 
with any degree of completeness. The above-mentioned observations are followed 
immediately by quite valueless considerations, as, for instance, that the ontogenetic 
and phyletic groups are in concentric ratio ! The author might as well speak of 
a quadrangular or triangular ratio ! 

P Facts of the same kind are also known in the Vascular Cryptogams, Muscineae , 
Characeae, Florideae, etc. — S. S.] 



FOUNDATION OF A THEORY OF HEREDITY. 207 

cannot be identical with that of the parent germ-cell. In order to 
prove this, it is only necessary to refer to the arguments as to the 
ontogenetic stages of the idioplasm. In the above-mentioned 
instances, the continuity from the germ-substance of the parent 
to that of the offspring can only be explained by the supposition 
that the somatic nucleoplasm still contains some unchanged germ- 
plasm. I believe that the fundamental idea of Jager and Nuss- 
baum is quite correct : it is the same idea which has led me to the 
hypothesis of the continuity of the germ-plasm, viz., the conviction 
that heredity can only be understood by means of such an hypothesis. 
But both these writers have worked out the idea in the form of an 
hypothesis which does not correspond with the facts. That this is 
the case is also shown by the following words of Nussbaum — ' the 
cell-material of the individual (somatic cells) can never produce a 
single sexual cell.' Such production undoubtedly takes place, not 
only in Hydroids and Phanerogams, but in many other instances. 
The germ-cells cannot indeed be produced by any indifferent cell 
of embryonic character, but by certain cells, and under circumstances 
which allow us to positively conclude that they have been pre- 
destined for this purpose from the beginning. In other words, 
the cells in question contain germ-plasm, and this alone enables 
them to become germ-cells. 

As a result of my investigations on Hydroids ^, I concluded that 
the germ-plasm is present in a very finely divided and therefore in- 
visible state in certain somatic cells, from the very beginning of 
embryonic development, and that it is then transmitted through 
innumerable cell-generations, to those remote individuals of the 
colony in which sexual products are formed. This conclusion is 
based upon the fact that germ-cells only occur in certain localized 
areas ('Keimstatten') in which neither germ-cells nor primitive 
germ-cells (the cells which are transformed into germ-cells at a later 
period) were previously present. The primitive germ-cells are also 
only formed in localized areas, arising from somatic cells of the 
ectoderm. The place at which germ-cells arise is the same in all 
individuals of the same species ; but differs in different species. It 
can be shown that such differences correspond to different phyletic 
stages of a process of displacement, which tends to remove the 

* Weismann, ' Die Entstehung der Sexualzellen bei den Hydromedusen.' Jena, 
1883. 



208 THE CONTINUITY OP THE GERM-PLASM AS THE 

localized area from its original position (the manubrium of the 
Medusa) in a centripetal direction. For the purposes of the present 
enquiry it is unnecessary to discuss the reasons for this change of 
position. The phyletic displacements of the localized areas are 
brought about during ontogeny by^j^n actual migration of primitive 
germ-cells from the place where they arose to the position at which 
they undergo differentiation into germ-cells. But we cannot believe 
that primitive germ-cells would migrate if the germ-cells could 
be formed from any of the other young cells of indifferent character 
which are so numerous in Hydroids. Even when the localized area 
undergoes very slight displacement, e. g. when it is removed from 
the exterior to the interior of the mesogloea ^, the change is always 
effected by active migration of primitive germ-cells through the 
substance of the mesogloea. Although the localized area has been 
largely displaced in the course of phylogeny, the changes in posi- 
tion have always taken place by very gradual stages, and never 
suddenly, and all these stages are repeated in the ontogeny of all 
existing species, by the migration of the primitive germ-cells from 
the ancestral ai'ea to the place where the germ-cells now arise. 
Hartlaub ^ has recently added a further instance (that of Obelia) to 
the numerous minute descriptions of these phyletic displacements of 
the localized area, and ontogenetic migrations of the primitive germ- 
cells, which are given in my work already referred to. The 
instance of Obelia is of especial interest as the direction of dis- 
placement is here reversed, taking place centrifugally instead of in 
a centripetal direction. 

But if displacements of the localized areas can only take place by 
the frequently roundabout method of the migi-ation of primitive 
germ-cells, we are obliged to conclude that such is the only manner 
in which the change can be effected, and that other cells are unable 
to play the role of the primitive germ-cells. And if other cells are 
unable to take this part, it must be because nucleoplasm of a 
certain character has to be present in order to form germ-cells, or 
according to the terms of my theory, the presence of germ-plasm is 

[* I adopt this term, suggested by E. Ray Lankester and G. C. Bourne, as the 
name of the supporting lamina of Coelenterata. See ' Quart. Joum. Microsc. Sci.* 
Jan. 1887, p. 28 .— E. B. P.] 

' Dr. Clemens Hartlaub, ' Ueber die Entetehung der Sexnalzellen bei Obelia.' 
Freiburg, Inaugural Dissertation : see also 'Zeitschrift fur wissenschaftliche Zoologie.' 
Bd. XLI. 1884. 



FOUNDATION OF A THEORY OF HEREDITY. 209 

indispensable for this purpose. I do not see how we can escape 
the conchision that there is continuity of the germ-plasm ; for 
if it were supposed that somatic idioplasm undergoes transforma- 
tion into germ-plasm, such an assumption would not explain 
why the displacement occurs \^ small stages, and with extreme 
and constant care for the preservation of a connexion with cells 
of the ancestral area. This fact can only be explained by the hypo- 
thesis that cell-generations other than those which end in the 
production of the cells of the ancestral area, are totally incapable 
of transformation into germ-cells. 

Strasburger has objected that the transmission of germ-plasm 
along certain lines, viz. through a certain succession of somatic 
cells, is impossible, because the idioplasm is situated in the nucleus 
and not in the cell-body, and because a nucleus can only divide 
into two exactly equal halves by the indirect method of division, 
which takes place, as we must believe, in these cases. ' It might 
indeed be supposed,' says Strasburger, 'that during nuclear division 
certain molecular groups remain unchanged in the nuclear sub- 
stance which is in other respects transformed, and that these 
groups are uniformly distributed through the whole organism ; 
but we cannot imagine that their transmission could only be effected 
along certain lines.' 

I do not think that Strasburger's objections can be maintained. 
I base this opinion on my previous criticism upon the assumed 
equality of the two daughter-nuclei formed by indirect division. 
I do not see any reason why the two halves must always possess 
the same structure, although they may be of equal size and weight. 
I am surprised that Strasburger should admit the possibility that 
the germ -plasm, which, as I think, is mixed with the idioplasm of 
the somatic cells, may remain unchanged in its passage through 
the body; for if this writer be correct in maintaining that the 
changes of nuclear substance in ontogeny are effected by the 
nutritive influence of the cell-body (cytoplasm), it follows that 
the whole nuclear substance of a cell must be changed at every 
division, and that no unchanged part can remain. We can only 
imagine that one part of a nucleus may undergo change while 
the other part remains unchanged, if we hold that the necessary 
transformations of nuclear substance are effected by purely internal 
causes, viz. that they follow from the constitution of the nucleo- 

P 



210 THE CONTINUITY OF THE GERM-PLASM AS THE 

plasm. But that one part may remain unchanged, and that such 
persistence dues, as a matter of fact, occur is shown by the cases 
above described, in which the g-erm-cells separate very early from 
the developing egg-cell. Thus in the egg of Diptera, the two 
nuclei which are first separated by division from the segmentation 
nucleus, fonn the sexual cells, and this proves that they receive 
the germ-plasm of the segmentation nucleus unchanged. But 
during or before the separation of these two nuclei, the remaining part 
of the segmentation nucleus must have become changed in nature, 
or else it would continue to form ' pole-cells ' at a later period 
instead of forming somatic cells. Although in many cases the 
cell-bodies of such early embryonic cells fail to exhibit any 
visible differences, the idioplasm of their nuclei must undoubtedly 
differ, or else they could not develope in different directions. It 
seems to me not only possible, but in every way probable, that the 
bodies of such early embryonic cells are equal in reality as well as 
in appearance ; for, although the idioplasm of the nucleus deter- 
mines the character of the cell-body, and although every differ- 
entiation of the latter depends upon a certain structure of its 
nucleoplasm, it does not necessarily follow that the converse pro- 
position is true, viz. that each change in the structure of the 
nucleoplasm must effect a change in the cell-body. Just as rain 
is impossible without clouds, but every cloud does not necessarily 
produce rain, so growth is impossible without chemical change, but 
chemical processes of every kind and degree need not produce 
growth. In the same manner every kind of change in the mole- 
cular structure of the nucleoplasm need not exercise a transforming 
influence on the cytoplasm, and we can easily imagine that a long 
series of changes in the nucleoplasm may appear only in the kind 
and energy of the nuclear divisions which take place, the cell- 
substance remaining unchanged, as far as its molecular and che- 
mical structure is concerned. This suggestion is in accordance 
with the fact that during the first period of embryonic develop- 
ment in animals, the cell-bodies do not exhibit any visible differ- 
ences, or only such as are very slight ; although exceptional in- 
stances occur, especially among the lower animals. But even 
these latter (e. g. the difference in appearance of the cells of the 
ectoderm and endoderm in sponges and Coelenterata) perhaps 
depend more largely upon a different admixture of nutritive sub- 



FOUNDATION OF A THEORY OF HEREDITY. 211 

stances than upon any marked difference in the cytoplasm itself. It 
is obvious that, in the construction of the embryo, the amount of 
cell-material must be first of all increased, and that it is only at a 
later period that the material must be differentiated so as to 
possess various qualities, according- to the principle of division of 
labour. Facts of this kind are also opposed to Strasbur goer's view, 
that the cause of changes in the nucleoplasm does not lie within 
this substance itself but within the cell-body. 

I believe I have shown that theoretically hardly any objections 
can be raised against the view that the nuclear substance of 
somatic cells may contain unchanged germ-plasm, or that this 
germ-plasm may be transmitted along certain lines. It is true 
that we might imagine a priori that all somatic nuclei contain 
a small amount of unchanged germ-plasm. In Hydroids such 
an assumption cannot be made, because only certain cells in a 
certain succession possess the power of developing into germ-cells ; 
but it might well be imagined that in some organisms it would 
be a great advantage if every part possessed the power of growing 
up into the whole organism and of producing sexual cells under 
appropriate circumstances. Such cases might exist if it were pos- 
sible for all somatic nuclei to contain a minute fraction of un- 
changed germ-plasm. For this reason, Strasburger's other objection 
against my theory also fails to hold ; viz. that certain plants can 
be propagated by pieces of rhizomes, roots, or even by means of 
leaves, and that plants produced in this manner may finally give 
rise to flowers, fruit and seeds, from which new plants arise. ' It 
is easy to grow new plants from the leaves of Begonia which a 
have been cut off and merely laid upon moist sand, and yet in the 
normal course of ontogeny the molecules of germ-plasm would not 
have been compelled to pass through the leaf; and they ought 
therefore to be absent from its tissue. Since it is possible to raise 
from the leaf a plant which produces flower and fruit, it is per- 
fectly certain that special cells containing the germ substance 
cannot exist in the plant.' But I think that this fact only proves, 
that in Begonia and similar plants, all the cells of the leaves 
or perhaps only certain cells contain a small amount of germ- 
plasm, and that consequently these plants are specially adapted 
for propagation by leaves. How is it then that all plants cannot 
be reproduced in this way ? No one has ever grown a tree from 

P 2 



212 THE CONTINUITY OP THE GERM-PLASM AS THE 

the leaf of the lime or oak, or a flowering- plant from the leaf 
of the tulip or convolvulus. It is insufficient to reply that, in the 
last-mentioned cases, the leaves are more strongly specialized, and 
have thus become unable to produce germ-substance ; for the leaf- 
cells in these different plants have hardly undergone histological 
differentiation in different degrees. If, notwithstanding, the one 
can produce a flowering plant, while the others have not this 
power, it is of course clear that reasons other than the degree 
of histological differentiation must exist ; and, according to my 
opinion, such a reason is to be found in the admixture of a minute 
quantity of unchanged germ-plasm with some of their nuclei. 

In Sachs' excellent lectures on the physiology of plants, we read 
on page 7 33 ^ — ' In the true mosses almost any cell of the roots, 
leaves and shoot-axes, and even of the immature sporogonium, 
may grow out under favourable conditions, become rooted, form 
new shoots, and give rise to an independent living plant.' Since 
such plants produce germ-cells at a later period, we have here 
a case which requires the assumption that all or nearly all cells 
must contain germ-plasm. 

The ttieory of the continuity of the germ-plasm seems to me 
to be still less disproved or even rendered improbable by the facts 
of the alternation of generations. If the germ-plasm may pass on 
from the egg into certain somatic cells of p,n individual, and if it 
can be further transmitted along certain lines, there is no difficulty 
in supposing that it may be transmitted through a second, third, 
or through any number of individuals produced from the former by 
budding. In fact, in the Hydroids, on which my theory of the 
continuity of the germ-plasm has been chiefly based, alternation 
of generations is the most important means of propagation. 

II. The Significance op the Polar Bodies. 

We have already seen that the specific nature of a cell depends 
upon the molecular structure of its nucleus ; and it follows from 
this conclusion that my theory is further, and as I believe strongly, 
supported, by the phenomenon of the expulsion of polar bodies, 
which has remained inexplicable for so long a time. 

• English translation, by H. Marshall Ward. Oxford, 1S87, Clarendon Press. 



FOUNDATION OF A THEORY OF HEREDITY. 213 

For if the specific molecular structure of a cell-body is caused 
and determined by the structure of the nucleoplasm, every kind 
of cell which is histologically differentiated must have a specific 
nucleoplasm. But the egg-cell of most animals, at any rate during 
the period of growth, is by no means an indifferent cell of the 
most primitive type. At such a period its cell-body has to 
perform quite peculiar and specific functions ; it has to secrete 
nutritive substances of a certain chemical nature and physical con- 
stitution, and to store up this food-material in such a manner 
that it may be at the disposal of the embryo during its develop- 
ment. In most cases the egg-cell also forms membranes which 
are often characteristic of particular species of animals. The 
growing egg-cell is therefore histologically differentiated : and 
in this respect resembles a somatic cell. It may perhaps be com- 
pared to a gland-cell, which does not expel its secretion, but 
deposits it within its own substance^. To perform such specific 
functions it requires a specific cell-body, and the latter depends 
upon a specific nucleus. It therefore follows that the growing 
egg-cell must possess nucleoplasm of specific molecular struc- 
ture, which directs the above-mentioned secretory functions of 
the cell. The nucleoplasm of histologically differentiated cells 
may be called histogenetic nucleoplasm, and the growing egg- 
cell must contain such a substance, and even a certain specific 
modification of it. This nucleoplasm cannot possibly be the same 
as that which, at a later period, causes embryonic development. 
Such development can only be produced by true germ-plasm 
of immensely complex constitution, such as I have previously 
attempted to describe. It therefore follows that the nucleus of 
the egg-cell contains two kinds of nucleoplasm : — germ-plasm 
and a peculiar modification of histogenetic nucleoplasm, which 
may be called ovogenetic nucleoplasm. This substance must greatly 
preponderate in the young egg-cell, for, as we have already seen, 
it controls the growth of the latter. The germ -plasm, on the 
other hand, can only be present in minute quantity at first, but 
it must undergo considerable increase during the growth of the 
cell. But in order that the germ-plasm may control the cell- 

[' Such gland-cells are known in both animals and plants. See W. Gardiner and 
Tokutaro Ito, On the structure of the mucilage-secreting cells of Blechnum occidentale 
L., and Osmunda regalis L., ' Annals of Botany,' vol. i. p. 49. — S. S.] 



214 THE CONTINUITY OP THE GERM-PLASM AS THE 

body, or, in other words, in order that embryonic development 
may begin, the still preponderating- ovogenetic nucleoplasm must 
be removed from the cell. This removal takes place in the same 
manner as that in which differing nuclear substances are separated 
during the ontogeny of the embryo : viz. by nuclear division, 
leading to cell-division. The expulsion of the polar bodies is 
nothing more than the . removal of ovogenetic nucleoplasm from 
the egg-cell. That the ovogenetic nucleoplasm continues to 
greatly preponderate in the nucleus up to the very last, may be 
cancluded from the fact that two successive divisions of the latter 
and the expulsion of two polar bodies appear to be the rule. If in 
this way a small part of the cell-body is expelled from the egg, 
the extrusion must in all probability be considered as an inevitable 
loss, without which the removal of the ovogenetic nucleoplasm 
cannot be eflPected. 

This is my theory of the significance of polar bodies, and I 
do not intend to contrast it, in extenso, with the theories pro- 
pounded by others ; for such theories are well known and differ 
essentially from my own. All writers agree in supposing that 
something which would be an obstacle to embiyonic development 
is removed from the egg ; but opinions differ as to the nature of 
this substance and the precise reasons for its removal ^. Some ob- 
servers (e. g. Minot ^, van Beneden, and Balfour) regard the 
nucleus as hermaphrodite, and assume that in the polar bodies the 
male element is expelled in order to render the egg capable of 
fertilization. Others speak of a rejuvenescence of the nucleus, 
others again believe that the quantity of nuclear substance must be 
reduced in order to become equal to that of the generally minute 
sperm-nucleus, and that the proportions for nuclear conjugation are 
in this way adjusted. 

The first view seems to me to be disproved by the fact that male 
as well as female qualities are transmitted by the egg-cell, while 
the sperm-cell also transmits female qualities. The germ-plasm of 
the nucleus of the egg cannot therefore be considered as female, 

' Thus in 1877 Btttschli thought that 'the chief significance of the formation of 
polar bodies lies in the removal of part of the nucleus of the egg, whether this 
removal is effected by simple expulsion or by the budding of the egg-cell.' ' Ent- 
wicklungsgeschichtliche Beitrage ; ' Zeitachrift fiir wissenschaftliche Zoologie, Bd. 
XXIX. p. 237, footnote. 

* C. S. Minot, ' Account, etc.;' Proc. Boston Soc. Nat. Hist. vol. xix. p. 165, 1877. 



FOUNDATION OF A THEORY OF HEREDITY. 215 

and that of the sperm-nucleus cannot be considered as male : both 
are sexually indifferent. The last view has been recently formulated 
by Strasburger, who holds that the quantity of the idioplasm 
contained in the germ-nucleus must be reduced by one half, and 
that a whole nucleus is ag-ain produced by conjugation with the 
sperm-nucleus. Although I believe that ^he fundamental idea 
underlying this hypothesis is perfectly correct, viz. that the in- 
fluence of each nucleus is as largely dependent upon its quantity 
as upon its quality, I must raise the objection that the decrease in 
quantity is not the explanation of the expulsion of polar bodies. The 
quantity of idioplasm contained in the germ-nucleus is, as a matter 
of fact, not reduced by one-half but by three-fourths, for two 
divisions take place one after the other. Thus by conjugation 
with the sperm-nucleus, which we may assume to be of the same 
size as the germ-nucleus, a nucleus is produced which can only 
contain half as much idioplasm as was present in the original 
germ-nucle«s, before division. Strasburger's view leaves un- 
explained the question why the size of the germ-nucleus, before the 
expulsion of polar bodies, was thus twice as large ; and even if we 
•neglect the theory of ovogenetic nucleoplasm and assume that this 
larger nucleus was entirely made up of germ-plasm, it must be 
asked why the egg did not commence segmentation earlier. The 
theory which explains the sperm-cell as the vitalizing principle 
which starts embryonic development, like the spark which kindles 
the gunpowder, would indeed answer this question in a very simple 
manner^ But Strasburger does not accept this theory, and holds, 
as I do, a very different view, which will be explained later on. 

If, on the other hand, we assume that the germ-nucleus contains 
two different kinds of nucleoplasm, the question is answered quite 
satisfactorily. In treating of parthenogenesis, further on, I shall 
mention a fact which seems to me to furnish a real proof of the 
validity of this explanation ; and, if we accept this fact for the 
present, it will be clear that the simple explanation now offered 
of phenomena which are otherwise so difficult to understand, 
would also largely support the theory of the continuity of the 
germ-plasm. Such an explanation would, above all, very clearly 
demonstrate the co-existence of two nucleoplasms with different 
qualities in one and the same nucleus. My theory must stand 
or fall with this explanation, for if the latter were disproved, the 



216 THE CONTINUITY OP THE QERM-PLA8M AS THE 

continuity of the germ- plasm could not be assumed in any instance, 
not even in the simplest cases, where, as in Diptera, the germ-cells 
are the first-formed products of embryonic development. For even 
in these insects the egg possesses a specific histological character 
which must depend upon a specifically differentiated nucleus. If 
then two kinds of nucleoplasm are not present, we must assume that 
in such cases the germ-plasm of the newly formed germ-cells, 
which has passed on unchanged from the segmentation nucleus, is at 
once transformed entirely into ovogenetic nucleoplasm, and must be 
re-transformed into germ-plasm at a later period when the egg is 
fully mature. We could not in any way understand why such a 
re-transformation requires the expulsion of part of the nuclear sub- 
stance. 

At all events, my explanation is simpler than all others, in that 
it only assumes a single transformation of part of the germ-plasm, 
and not the later re-transformation of ovogenetic nucleoplasm into 
germ-plasm, which is so improbable. The ovogenetic nucleoplasm 
must possess entirely different qualities from the germ-plasm ; and, 
above all, it does not readily lead to division, and thus we can better 
understand the fact, in itself so remarkable, that egg-cells do not 
increase in number by division, when they have assumed their 
specific structure, and are controlled by the ovogenetic nucleoplasm. 
The tendency to nuclear division, and consequently to cell-division, 
is not produced until changes have to a certain extent taken place 
in the mutual relation between the two kinds of nucleoplasm 
contained in the germ-nucleus. This change is coincident with 
the attainment of maximum size by the body of the egg-cell. 
Strasburger, supported by his observations on Spirogyra, concludes 
that the stimulus towards cell-division emanates from the cell- 
body; but the so-called centres of attraction at the poles of the 
nuclear spindle obviously arise under the influence of the nucleus 
itself, even if we admit that they are entirely made up of cytoplasm. 
But this point has not been decided upon, and we may presume 
that the so-called ' Polkorperchen ' of the spindle (Fol) are derived 
from the nucleus, although they are placed outside the nuclear 
membrane ^. Many points connected with this subject are still in a 

* E. van Beneden and Boveri have recently, quite independently of each other, 
made a more exact study of these ' Polkorperchen ' (* Centrosoma,' Boveri). They 
show that nuclear division starts from these bodies, although the mode of origin of 
the latter is not yet quite clear. — A. W., 1888. 



FOUNDATION OF A THEORY OF HEREDITY. 217 

state of uncertainty, and we must abstain from general conclusions 
until it has been possible to demonstrate clearly the precise nature 
of certain phenomena attending- indirect nuclear division, which 
still remain obscure in spite of the efforts of so many excellent 
observers. We cannot even form a decided opinion as to whether 
the chromatin or the achromatin of the nuclear thread is the real 
idioplasm. But although these points are not yet thoroughly 
understood, we are justified in maintaining that the cell enters 
upon division under the influence of certain conditions of the 
nucleus, although the latter are invisible until cell-division has 
ah'eady commenced. 

I now pass on to examine my hypothesis as to the significance of 
the formation of polar bodies, in the light of those ascertained facts 
which bear upon it. 

If the expulsion of the polar bodies means the removal of the 
ovogenetic nucleoplasm after the histological differentiation of the 
egg- cell is complete, we must expect to find polar bodies in all 
species except those in which the egg-ceU has remained in a 
primitive undifferentiated condition, if indeed such species exist. 
Wherever the egg-cell assumes the character of a specialized cell, 
e. g. in the attainment of a particular size or constitution, in the 
admixture of food-yolk, or the formation of membranes, it must also 
contain ovogenetic nucleoplasm, which must ultimately be removed 
if the germ-plasm is to gain control over the egg-cell. It does not 
signify at all, in this respect, whether the egg is or is not destined 
for fertilization. 

If we examine the Metazoa in regard to this question, we find 
that polar bodies have not yet been discovered in sponges ^, but this 
negative evidence is no proof that they are really absent. In all 
probability, no one has ever seriously endeavoured to find them, and 
there are perhaps difficulties in the way of the proofs of their exist- 
ence, because the egg-cell lies free for a long time and even moves 
actively in the tissue of the mesogloea. We might expect that the 
formation of polar bodies takes place here, as in all other instances, 
when the egg becomes mature, that is, at a time when the eggs 
are already closely enveloped in the sponge tissue. At all events 
the eggs of sponges, as far as they are known, attain a specific 

* The existence of polar bodies in sponges has been i-eceutly proved by Fiedler : 
Zool., Anzeiger., Nov. 28, 1887.— A. W., 1888. 



218 THE CONTlKUITY OF THE GERM-PLASM AS THE 

nature, in the possession of a peculiar cell-body, frequently con- 
taining food-yolk, and of the nucleus which is characteristic of all 
animal eggs during* the process of growth. Hence we cannot 
doubt the presence of a specific ovogenetic nucleoplasm, and must 
therefore also believe that it is ultimately removed in the polar 
bodies. 

In other Coelenterata, in worms, echinoderms, and in molluscs 
polar bodies have been described, as well as in certain Crustacea, 
viz. in Balanus by Hoek and in Cetochilus septentrional e by Grobben. 
The latter instance appears to be quite trustworthy, but there is 
some doubt as to the former and also as regards Moina (a Daphnid), 
in which Grobben found a body, which he considered to be a polar 
body, on the upper pole of an egg which was just entering upon 
segmentation. In insects polar bodies have not been described up 
to the present time ^, and only in a few cases in Vertebrata, as in 
Fetromt/zon by Kupffer and Benecke. 

It must be left to the future to decide whether the expulsion of 
polar bodies occurs in those large groups of animals in which they 
have not been hitherto discovered. The fact, however, that they 
have not been so discovered cannot be urged as an objection to my 
theory, for we do not know a priori whether the removal of the ovo- 
i genetic nucleoplasm has not been effected in the course of phylogeny 
in some other and less conspicuous manner. The cell-body of the 
polar globules is so minute in many eggs that it was a long time 
before the cellular nature of these structures was recognized ^ ; and 
it is possible that their minute size may point to the fact that 
a phyletic process of reduction has taken place, to the end that the 
egg may be deprived of as little material as possible. It is at all 
events proved that in all Metazoan groups the nucleus undergoes 
changes during the maturation of the egg, which are entirely similar 
to those which lead to the formation of polar bodies in those eggs 
which possess them. In the former instances it is possible that 
nature has taken a shortened route to gain the same end. 

It would be an important objection if it could be shown that no 

^ They have now been observed in many species, so that their general occurrence 
in insects is tolerably certain. Compare bibliography given in Weismann and 
Ischikawa, ' Weitere Untersuchungen zum Zablengesetz der Bichtungskorper,' 
'Zoolog. Jahrbiicher,' vol. iii. 1888, p. 593.— A. W., 1888. 

' Van Beneden, even in his last work, considers these bodies to have'only the value 
of nuclei ; 1. c, p. 394. 



FOUNDATION OF A THEORY OF HEREDITY. 219 

process corresponding to the expulsion of polar bodies takes place 
in the male g-erm-cells, for it is obvious that here also we should, 
according" to my theory, expect such a process to occur. The great 
majority of sperm-cells differ so widely in character from the ordi- 
nary indifferent (i. e. undifferentiated) cells, that they are evidently 
histologically differentiated in a very high degree, and hence the 
sperm-cells, like the yolk-forming germ-cells, must possess a specific 
nuclear substance. The majority of sperm-cells therefore resemble 
the somatic cells in that they have a specific histological structure, 
but their characteristic form has nothing to do with their fertilizing 
power, viz. with their power of being the bearers of germ-plasm. 
Important as this structure is, in order to render it possible that 
the egg-cell may be approached and penetrated, it has nothing to 
do with the property of the sperm-cell to transmit the qualities of 
the species and of the individual to the following generation. The 
nuclear substance which causes such a cell to assume the appearance 
of a thread, or a stellate form (in Crustacea), or a boomerang form 
(present in certain Daphnids), or a conical bullet shape (Nematodes), 
cannot possibly be the same nuclear substance as that which, after 
conjugation with the egg-cell, contains in its molecular > structure 
the tendency to build up a new Metazoon of the same kind as that 
by which it was produced. We must, therefore, conclude that the 
sperm-cell also contains two kinds of nucleoplasm, namely, germ- 
plasm and spermogenetic nucleoplasm. 

It is true that we cannot say a jmori whether the influence 
exercised on the sperm-cell by the spermogenetic nucleoplasm 
might not be eliminated by some means other than its removal 
from the cell. It is conceivable, for instance, that this substance 
may be expelled from the nucleus, but may remain in the cell-body, 
where it is in some way rendered powerless. We do not yet really 
know anything of the essential conditions of nuclear division, and 
it is qxaite impossible to bring forward any facts in support of my 
previous suggestion. The germ-plasm is supposed to be present 
in the nucleus of the growing egg-cell in smaller quantity than the 
ovogenetic nucleoplasm, and the germ-plasm gradually increases in 
quantity: thus when the egg has attained its maximum size, the 
opposition between the two different kinds of nucleoplasm becomes 
so marked, in consequence of the alteration in their quantitative 
relations, that their separation, viz. nuclear division, results. But 



220 THE CONTINUITY OF THE GERM-PLASM AS THE 

although we are not able to distinguish, by any visible charac- 
teristics, the different kinds of nucleoplasm which may be united 
in one nuclear thread, the assumption that the influence of each 
kind bears a direct proportion to its quantity is the most ob\iou9 
and natural one. The tendency of the germ-plasm contained 
in the nucleus cannot make itself felt so long as an excess of 
ovogenetic nucleoplasm is also present. We may imagine that 
the effects of the two different kinds of nucleoplasm are combined 
to produce a resultant effect ; but when the two influences exerted 
upon the cell are nearly opposed, only the stronger can make 
itself felt, and in such a ease the latter must exceed the former in 
quantity, because part of it is as it were neutralized by the other 
nucleoplasm working in an opposite direction. This metaphorical 
representation may give us a clue to explain the fact that the 
ovogenetic nucleoplasm comes to exceed the germ-plasm in quan- 
tity. For obviously these two kinds of nucleoplasm exert oppo- 
site tendencies in at least one respect. The germ-plasm tends 
to effect the division of the cell into the two first segmentation 
spheres ; the ovogenetic nucleoplasm, on the other hand, possesses a 
tendency towards the growth of the cell-body without division. 
Hence the germ-plasm cannot make itself felt, and is not able to 
expel the ovogenetic nucleoplasm until it has reached such a 
relative size as enables it to successfully oppose the latter. 

Applying these ideas to the sperm-cells we must see whether 
the expulsion of part of the nuclear substance, viz. of the spermo- 
genetic nucleoplasm, corresponding to the ovogenetic nucleoplasm, 
takes place in them also. 

As far as we can judge from thoroughly substantiated obser- 
vations such phenomena are indeed found in many cases, although 
they appear to be different from those occurring in the egg-cell, 
and cannot receive quite so certain an interpretation. 

The attempt to prove that a process similar to the expulsion of 
polar bodies takes place in the formation of sperm-cells has already 
been made by those observers who regard such expulsion as the 
removal of the male element from the egg, thus leading to sexual 
differentiation ; for such a theory also requires the removal of part 
of the nuclear substance from the maturing sperm-cell. Thus, 
according to E. van Beneden and Ch. Julin, the cells which, in 
Ascaris, produce the spermatogonia (mother-cells of the sperm-cells). 



FOUNDATION OF A THEORY OF HEREDITY. 221 

expel certain elements from their nuclear plate, a phenomenon which 
has not been hitherto observed in any other animal, and even in 
this instance has only been inferred and not directly observed. 
Moreover the sperm-cells have not attained their specific form- 
(conical bullet- shaped) at the time when this expulsion takes place 
from the spermatogonia, and we should expect that the spermo- 
genetic nucleoplasm would not be removed until it has copapleted 
its work, viz. not until the specific shape of the sperm-cell has 
been attained. We might rather suppose that phenomena explic- 
able in this way are to be witnessed in those sperm-blastophores 
(mother-cells of sperm-cells) which, as has been known for a long 
time, are not employed in the formation of the nuclei of sperm- 
cells, but for the g-reater part remain at the base of the latter and 
perish after their maturation and separation. In this case an in- 
fluence might be exerted by these nuclei upon the specific form of 
the sperm-cells, for the former arise and develope in the form of 
bundles of spermatozoa in the interior of the mother-cell. 

It has been already shown in many groups of animals that parts 
of the sperm-mother-cells ^ perish, without developing into sperm- 
cells, as in Selachians, in the frog, in many worms and snails, 
and also in mammals (Blomfield), But the attention of observers 
has been directed to that part of the cell-body which is not used 
in the formation of sperm-cells, rather than to the nucleus ; and 
the proof that part of the nucleus also perishes is still wanting 
in many of these cases. Fresh investigation must decide whether 
the nucleus of the sperm-mother-cell perishes as a general rule, 
and whether part of the nucleus is rendered powerless in some 
other way, where such mother-cells do not exist. Perhaps the 
paranucleus (Nebenkern) of the sperm-cell, first described by La 
Valette St, George, and afterwards found in many animals of very 
different groups, is the analogue of the polar body. It is true that 
this so-called paranucleus is now considered as a condensed part of 

* I purposely abstain from using a more precise term, for the complicated ter- 
minology employed in spermatogenesis hardly contributes anything to the elucida- 
tion of the phenomena themselves. Why do we not simply speak of sperm-cells 
and spermatoblasts, and distinguish the latter by numbers when they occur in 
successive generations of different form? Moreover, all the names which have been 
suggested for successive stages of development, can only be applied to the special 
group of animals upon which the observations have been made. Hence great con- 
fusion results from the use of such terms as spermatoblasts, spermatogonia, sperma- 
tomeres, spermatooysts, spermatocytes, spermatogemmae, etc. 



222 THE CONTINUITY OF THE GERM-PLASM AS THE 

tlie cell-body, but we must remember that it has be^ hitherto A 
question whether the head of the spermatozoon is formed from the 
nucleus of the cell or from the paranucleus ; and that the observers 
who held the former view were in consequence obliged to regard 
the paranucleus as a product of the cell-body. But according to 
the most recent investigations of FoP, Roule^, Balbiani^, and Will*, 
upon the formation of the follicular epithelium in the ovary of 
different groups, it is not improbable that parts of the nucleus 
may become detached without passing through the process of 
karyokinesis. Thus it is very possible that the paranucleus may 
be a product of the main nucleus and not a condensed part of 
the cell-body. This view is supported by its behaviour with stain- 
ing reagents, while the other view, that it arises from the cell- 
substance, is not based upon direct observation. Consequently 
future investigation must decide whether the paranucleus is to 
be considered as the spermogenetic nucleoplasm expelled from the 
nucleus. But even if this question is answered in the affirmative, 
we should still have to explain why this nuclear substance, remain- 
ing in the cell-body, does not continue to exercise any control over 
the latter. 

Strasburger has recently enumerated a large number of cases 
from different groups of plants, in which the maturation of both 
male and female germ-cells is accompanied by phenomena similar 
to the expulsion of polar bodies. In this respect the phenomena 
occurring in the pollen-grains of Phanerogams bear an aston- 
ishing resemblance to the maturation of the animal egg. For 
instance, in the larch, the sperm-mother-cell divides three times 
in succession, the products of division being very unequal on each 
occasion ; and exactly as in the case of polar bodies, the three small 
so-called vegetative cells shrink rapidly after separation, and have 
no further physiological value. According to Strasburger, the so- 
called 'ventral canal-cell,' which, in mosses, ferns, and Conifers, 

* Fol, ' Sur I'origine des cellules du follicule et de I'oviile chez les Ascidies.' 
Compt. rend., 28 mai, 1883. 

''' Koule, 'La structure de I'ovaire et la formation des oea& chez les Phallusiadees.' 
Ibid., 9 avril, 1883. 

- * Balbiani, ' Sur I'origine des cellules du follicule et du noyau vitellin de I'oeuf 
chez lea Geophiles.' Zool. Anzeiger, 1883, Nos. 155, 156. 

* Will, ' Ueber die Entstehung des Dotters und der Epithelzellen bei den Amphi- 
bien und Insecten.' Ibid., 1884, Nos. 167, 168. 



FOUNDATION OF A THEORY OF HEREDITY. 223 

separates from the female germ-cell, reminds us, in every way, of 
the polar bodies of animal eggs. Furthermore, the spermatozoids 
in the mosses and vascular cryptogams throw off a small vesicle 
before performing their functions ^. On the other hand the equiva- 
lents of ' polar bodies ' (the ' ventral canal-cells ') are said to be ab- 
sent in the Cycads, although these are so nearly allied to Conifers. 
Furthermore, 'no phenomenon occurs in the oospheres (ova) of An- 
giosperms which can be compared to the formation of polar bodies.' 
Strasburger therefore concludes that the separation of certain parts 
from the germ-cells is not in all cases necessary for maturation, 
and that such phenomena are not fundamental, like those of 
fertilization, which must always take place along the same morpho- 
logical lines. He further concludes that the former phenomena are 
only necessary in the case of the germ-cells of certain organisms, 
in order to bring the nuclei destined for the sexual act into the 
physiological condition necessary for its due performance. 

I am unwilling to abandon the idea that the expulsion of the 
histogenetic parts of the nuclear substance, dming the maturation 
of germ-cells, is also a general phenomenon in plants ; for the 
process appears to be fundamental, while the argument that it 
has not been proved to occur universally is only of doubtful value. 
The embryo-sac of Angiosperms is such a complex structure that 
it seems to me to be possible (as it does to Strasburger) that ' pro- 
cesses which precede the formation of the egg-cell have borne 
relation to the sexual differentiation of the nucleus of the egg.' 
Besides, it is possible that the vegetable egg-cell may, in certain 
cases, possess so simple a structure and so small a degree of histo- 
logical specialization, that it would not be necessary for it to 
contain any specific histogenetic nucleoplasm : thus it would con- 
sist entirely of germ-plasm from the first. In such cases, of course, 
its maturation would not be accompanied by the expulsion of 
somatic nucleoplasm. 

I have hitherto abstained from discussing the question as to 
whether the process of the formation of polar bodies may require 
an interpretation which is entirely different from that which I 
have given it, whether it may receive a purely morphological inter- 

[} It is almost certain that this vesicle is not derived from the nucleus, but from 
the cytoplasm of the sperm-mother-cell. See Douglas H. Campbell, 'Zur Ent- 
wicklungsgeschichte der Spermatozoiden ' in Berichte der deutschen botanischen 
Gesellschaft, vol. v, 1887, p. 122.— S. S.] 



224 THE CONTINUITY OF THE GERM-PLASM AS THE 

pretation. In former times it could only be regarded as of purely 
phyleticsig'nificance : it could only be looked upon as the last remnant 
of a process which formerly possessed some meaning-, but which is 
now devoid of any physiological importance. We are indeed com- 
pelled to admit that a process does occur in connexion with the true 
polar bodies of animal eggs, which we cannot explain on physio- 
logical grounds ; I mean the division of the polar bodies after they 
have been expelled from the egg. In many animals the two polar 
bodies divide again after their expulsion, so as to form four bodies, 
which distinctly possess the structure of cells, as Trinchese observed 
in the case of gastropods. But, in the first place, this second division 
does not always take place, and, secondly, it is very improbable 
that a process which occurs during the first stage of ontogeny, 
or more properly speaking, before the commencement of ontogeny, 
and which is, therefore, a remnant of some excessively ancient 
phyletic stage, would have been retained up to the present day 
unless it possessed some very important physiological significance. 
We may safely maintain that it would have disappeared long ago 
if it had been without any physiological importance. Relying 
on our knowledge of the slow and gradual, although certain, dis- 
appearance, in the course of phylogeny, of organs which have lost 
their functions, and of processes which have become meaningless, 
we are compelled to regard the process of the formation of polar 
bodies as of high physiological importance. But this view does 
not exclude the possibility that the process possessed a morpho- 
logical meaning also, and I believe that we ai'e quite justified in 
attempting (as Biitsehli ^ has recently done) to discover what this 
morphological meaning may have been. 

Should it be finally proved that the expulsion of polar bodies 
is nothing more than the removal of histogenetic nucleoplasm 
from the germ-cell, the opinion (which is so intimately connected 
with the theory of the continuity of the germ-plasm) that a re- 
transfoiination of specialised idioplasm into germ-plasm cannot 
occur, would be still further confirmed ; for we do not find that any 
part of an organism is thrown away simply because it is useless : 
organs that have lost their functions are re-absorbed, and their 
material is thus employed to assist in building up the organism. 

' Biitachli, ' Gedanken uber die morphologische Bedeutung der sogenannten Rich- 
tungskorperchen,' Biolog. Centralblatt, Bd. VI. p. 5, 1884. 



FOUNDATION OF A THEORY OF HEREDITY. 225 



III. On the Nature of Parthenogenesis. 

It is well known that the formation of polar bodies has been 
repeatedly connected with the sexuality of germ-cells, and that it has 
been employed to explain the phenomena of parthenogenesis. I 
may now, perhaps, be allowed to develope the views as to the 
nature of pai"thenogenesis at which I have arrived under the in- 
fluence of my explanation of polar bodies. 

The theory of parthenogenesis adopted by Minot and Balfour is 
distinguished by its simplicity and clearness, among all other in- 
terpretations which had been hitherto offered. Indeed, their ex- 
planation follows naturally and almost as a matter of course, if the 
assumption made by these observers be correct, that the polar 
body is the male part of .the hermaphrodite egg-cell. An egg 
which has lost its male part cannot develope into an embryo until 
it has received a new male part in fertilization. On the other 
hand, an egg which does not expel its male part may develope with- 
out fertilization, and thus we are led to the obvious conclusion that 
parthenogenesis is based upon the non-expulsion of polar bodies. 
Balfour distinctly states ' that the function of forming polar cells 
has been acquired by the ovum for the express purpose of prevent- 
ing parthenogenesis ^' 

It is obvious that I cannot share this opinion, for I regard the ) 
expulsion of polar bodies as merely the removal of the ovogenetic 
nucleoplasm, on which depended the development of the specific 
histological structure of the egg-cell. I must assume that the 
phenomena of maturation in the parthenogenetic egg and in the 
sexual egg are precisely identical, and that in both, the ovogenetic 
nucleoplasm must in some way be removed before embryonic de- 
velopment can begin. 

Unfortunately the actual proof of this assumption is not so com- 
plete as might be desired. In the first place, we are as yet uncer- 
tain whether polar bodies are or are not expelled by parthenogenetic 
eggs ^ ; for in no single instance has such expulsion been established 
beyond doubt. It is true that this deficiency does not afford any 

* F. M. Balfour, ' Comparative Embryology,' vol. i. p. 63. 

^ The formation of a polar body in parthenogenetic eggs has now been proved : see 
note at the end of this Essay; see also Essay VI. — A. W., 1888. 



226 THE CONTINUITY OF THE GERM-PLASM AS THE 

support to the explanation of Minot and Balfour, for in all cases 
in which polar bodies have not been found in parthenogenetic eggs, 
! these structures are also absent from the egg-s which require fertiliza- 
tion in the same species. But although the expulsion of polar 
bodies in parthenogenesis has not yet been proved to occur, we must 
assume it to be nearly certain that the phenomena of maturation, 
whether connected or unconnected with the expulsion of polar 
bodies, are the same in the eggs which develope parthenogenetically 
and in those which are capable of fertilization, in one and the same 
species. This conclusion depends, above all, upon the phenomena 
of reproduction in bees, in which, as a matter of fact, the same egg 
may be fertilized or may develope parthenogenetically, as I shall 
have occasion to describe in greater detail at a later period. 

Hence when we see that the eggs of many animals are capable of 
developing without fertilization, while in other animals such de- 
velopment is impossible, the difference between the two kinds of 
eggs must rest upon something more than the mode of transforma- 
tion of the nucleus of the germ-cell into the first segmentation 
nucleus. There are, indeed, facts which distinctly point to the con- 
clusion that the difference is based upon quantitative and not 
qualitative relations. A large number of insects are exceptionally 
reproduced by the parthenogenetic method, e.g. in Lepidoptera. 
Such development does not take place in all the eggs laid by 
an unfertilized female, but only in part, and generally a small 
fraction of the whole, while the rest die. But among the latter 
there are some which enter upon embryonic development without 
being able to complete it, and the stage at which development 
may cease also varies. It is also known that the eggs of higher 
animals may pass through the first stages of segmentation without 
having been fertilized. This was shown to be the case in the egg 
of the frog by Leuckart ^, in that of the fowl by Oellacher ^, and 
even in the egg of mammals by Hensen ^. 

Hence in such cases it is not the impulse to development, but the 

' R. Leuckart, — article * Zeugiing,' in R. Wagner's ' Handworterbuch der Phy- 
siologic,' 1853, Bd. IV. p, 958. Similar observations were made by Max Schultze. 
These observations appear however to be erroneous, for PflOger has since shown that 
the eggs of frogs never develope if the necessary precautions are taken to prevent the 
access of any spermatozoa to the water. — A. W., 1888. 

* Oellacher, ' Die Veriinderungen des unbefruchteten Keims des Hiihncheneies. 
' Zeitschrift fur wissenschaftliche Zoologie,' Bd. XXII. p. 181. 1872. 

' Hensen, ' Centralblatt,' 1869, No. 26. 



FOUNDATION OF A THEORY OF HEREDITY. 227 

power to complete it, which is absent. We know that force is 
always bound up with matter, and it seems to me that such 
instances are best explained by the supposition that too small an 
amount of that form of matter is present, which, by its controlling 
ag-ency, effects the building'-up of the embryo by the transforma- 
tion of mere nutritive material. This substance is the germ-plasm 
of the segmentation nucleus, and I have assumed above that it is 
altered in the course of ontogeny by changes which arise from 
within, so that, when sufficient nourishment is afforded by the cell- 
body, each succeeding stage necessarily results from the preceding 
one. I believe that changes arise in the constitution of the 
nucleoplasm at each cell-division which takes place during the 
building-up of the embryo, changes which either correspond or 
diflPer in the two halves of each nucleus. If, for the present, we 
neglect the minute amount of unchanged germ-plasm which is 
reserved for the formation of the germ-cells, it is clear that a great 
many different stages in the development of somatic nucleoplasm 
are thus formed, which may be denominated as stages i, a, 3, 4, &c., 
up to n. In each of these stages the cells differ more as develop- 
ment proceeds, and as the number by which the stage is denomi- 
nated becomes higher. Thus, for instance, the two first segmen- 
tation spheres wouM represent the first stage of somatic nucleo- 
plasm, a stage which may be considered as but slightly different 
in its molecular structure from the nucleoplasm of the segmentation 
nucleus ; the four first segmentation spheres would represent the 
second stage ; the succeeding eight spheres the third, and so on. It 
is clear that at each successive stage the molecular structure of the 
nucleoplasm must be further removed from that of the germ-plasm, 
and that, at the same time, the cells of each successive stage must 
also diverge more widely among themselves in the molecular 
structure of their nucleoplasm. Early in development each cell 
must possess its own peculiar nucleoplasm, for the further course of 
development is peculiar to each cell. It is only in the later stages 
that equivalent or nearly equivalent cells are formed in -large 
numbers, cells in which we must also suppose the existence of 
equivalent nucleoplasm. 

If we may assume that a certain amount of germ-plasm must be 
contained in the segmentation nucleus in order to complete the 
whole process of the ontogenetic differentiation of this substance ; 

Q 3 



228 THE CONTINUITY OP THE GERM-PLASM AS THE 

if we may further assume that the quantity of germ-plasm in the 
seg-mentation nucleus varies in different cases ; then we should be 
able to understand why one egg can only develope after fertiliza- 
tion, while another can beg-in its development without fertilization, 
but cannot finish it, and why a third is even able to complete its 
development. We should also understand why one egg- only passes 
through the fii'st stages of segmentation and is then arrested, while 
another, reaches a few more stages in advance, and a third de- 
velopes so far that the embryo is nearly completely formed. These 
differences would depend upon the extent to which the germ-plasm, 
originally present in the egg, was sufficient for the development of 
the latter ; development will be arrested as soon as the nucleoplasm 
is no longer capable of producing the succeeding stage, and is thus 
unable to enter upon the following nuclear division. 

From a general point of view such a theory would explain many 
difficulties, and it would render possible an explanation of the 
phyletic origin of parthenogenesis, and an adequate understanding 
of the strange and often apparently abrupt and arbitrary manner 
of its occurrence. In my works on Daphnidae I have already laid 
especial stress upon the proposition that parthenogenesis in insects 
and Crustacea certainly cannot be an ancestral condition which has 
been transmitted by heredity, but that it has been derived from a 
sexual condition. In what other way can we explain the fact that 
parthenogenesis is present in certain species or genera, but absent 
in others closely allied to them ; or the fact that males are entirely 
wanting in species of which the females possess a complete apparatus 
for fertilization ? I will not repeat all the arguments with which 
I attempted to support this conclusion ^. Such a conclusion may 
be almost certainly accepted for the Daphnidae, because partheno- 
genesis does not occur in their still living ancestors, the Phyllo- 
pods, and especially the Estheridae. In Daphnidae the cause and 
object of the phyletic development of parthenogenesis may be traced 
more clearly than . in any other group of animals. In Daphnidae 
we can accept the conclusion with greater certainty than in all 
other groups, except perhaps the Aphid ae, that parthenogenesis is 
extremely advantageous to species in certain conditions of Hfe ; and 
that it has only been adopted when, and as far as,' it has been 

* Weismann, 'Beitrage zur Naturgeschichte der Daphnoiden,' Leipzig, 1876-79, 
Abhandlung VII, and ' Zeitschrift fur wissenachaftliche Zoologie,' Bd. XXXIII. 



FOUNDATION OF A. THEORY OF HEREDITY. 229 

beneficial ; and further, that at least in this group parthenogenesis 
became possible, and was adopted, in each species as soon as it 
became useful. Such a result can be easily understood if it is only 
the presence of more or less germ-plasm which decides whether an 
egg is, or is not, capable of development without fertilization. 

If we now examine the foundations of this hypothesis we shall 
find that we may at once accept one of its assumptions, viz. that 
fluctuations occur in the quantity of germ-plasm in the segmen- 
tation nucleus; for there can never be absolute equality in any 
single part of different individuals. As soon therefore as these 
fluctuations become so great that parthenogenesis is produced, it may 
become, by the operation of natm'al selection, the chief mode of 
reproduction of the species or of certain generations of the species. 
In order to place this theory upon a firm basis, we have simply to 
decide whether the quantity of germ-plasm contained in the seg- 
mentation nucleus is the factor which determines development ; 
although for the present it will be suflScient if we can render this 
view to some extent probable, and show that it is not in contra- 
diction with established facts. 

At first sight this hypothesis seems to encounter serious difii- 
culties. It will be objected that neither the beginning nor the end 
of embryonic development can possibly depend upon the quantity 
of nucleoplasm in the segmentation nucleus, since the amount may 
be continually increased by growth ; for it is well known that 
during embryonic development the nuclear substance increases 
with astonishing rapidity. By an approximate calculation I found ^ 
that, in the egg of a Cynlps, the quantity of nuclear substance 
present at the time when the blastoderm was about to be formed, 
and when there were twenty-six nuclei, was even then seven times 
as great as the quantity which had been contained in the seg- 
mentation nucleus. How then can we imagine that embryonic 
development would ever- be arrested from want of nuclear sub- 
stance, and if such deficiency really acted as an arresting force, how 
then could development begin at all ? We might suppose that 
when germ-plasm is present in sufficient quantity to start segmen- 
tation, it must also be sufficient to complete the development ; for 
it grows continuously, and must presumably always possess a power 

* Weismann, 'Beitrage zur Kenntniss der ersten Entwicklungsvorgange im 
Insectenei,' Bonn, 1882, p. 106. 



230 THE CONTINUITY OF THE GERM-PLASM AS THE 

equal to that which it possessed at the beginning, and which was 
just sufficient to start the process of segmentation. If at each 
ontogenetic stage, the quantity of nucleoplasm is just sufficient to 
produce the following stage, we might well imagine that the whole 
ontogeny would necessarily be completed. 

The flaw in this argument lies in the erroneous assumption that 
the growth of nuclear substance is, when the quality of the nucleus 
and the conditions of nutrition are equal, unlimited and un- 
controlled. The intensity of growth must depend upon the quan- 
tity of nuclear substance with which growth and the phenomena of 
segmentation commenced. There must be an optimum quantity 
of nucleoplasm with which the growth of the nucleus proceeds 
most favourably and rapidly, and this optimum will be represented 
in the normal size of the segmentation nucleus. Such a size 
is just sufficient to produce, in a certain time and under certain 
external conditions, the nuclear substance necessary for the construc- 
tion of the embryo, and to start the long series of cell-divisions. 
When the segmentation nucleus is smaller, but large enough to 
enter upon segmentation, the nuclei of the two first embryonic 
cells will fall rather more below the normal size, because the 
growth of the segmentation nucleus during and after division will 
be less rapid on account of its unusually small size. The succeed- 
ing generations of nuclei will depart more and more from the 
normal size in each respective stage, because they do not pass into 
a resting-stage during embryonic development, but divide again 
immediately after their formation. Hence nuclear growth would 
become less vigorous as the nuclei fell more and more below the 
optimum size, and at last a moment would arrive when they would 
be unable to divide, or would be at least unable to control the cell- 
body in such a manner as to lead to its division. 

The first event of importance for embryonic development is the 
maturation of the egg, i. e. the transformation of the nucleus of the 
germ-cell into a nuclear spindle and the removal of the ovogenetic 
nucleoplasm by the separation of polar bodies, or by some ana- 
logous process. There must be some cause for this separation, and 
I have already tried to show that it may lie in the quantitative 
relations which obtain between the two kinds of nucleoplasm con- 
tained in the nucleus of the egg. I have suggested that the 
germ -plasm, at first small in quantity, undergoes a gradual increase. 



FOUNDATION OF A THEORY OF HEREDITY. 231 

80 that it can finally oppose the ovogenetie nucleoplasm. I will not 
further elaborate this suggestion, for the ascertained facts are in- 
su£Rcient for the purpose. But the appearances witnessed in nuclear 
division indicate that there are opposing forces, and that such a 
contest is the motive cause of division ; and Roux ^ may be right 
in referring the opposition to electrical forces. However this may 
be, it is perfectly certain that the development of this opposition 
is based upon internal conditions arising during growth in the 
nucleus itself. The quantity of nuclear thread cannot by itself 
determine whether the nucleus can or cannot enter upon division ; 
if so, it would be impossible for two divisions to follow each other 
in rapid succession, as is actually the case in the sepai-ation of 
the two polar bodies, and also in their subsequent division. In 
addition to the effects of quantity, the internal conditions of the 
nucleus must also play an important part in these phenomena. 
Quantity alone does not necessarily produce nuclear division, or the 
nucleus of the egg would divide long before maturation is complete, 
for it contains much more nucleoplasm than the female pronucleus, 
which remains in the egg after the expulsion of the polar bodies, 
and which is in most cases incapable of further division. But the 
fact that segmentation begins immediately after the conjugation of 
male and female pronuclei, also shows that quantity is an essential 
requisite. The effect of fertilization has been represented as ana- 
logous to that of the spark which kindles the gunpowder. In the 
latter case an explosion ensues, in the former segmentation begins. 
Even now, many authorities are inclined to refer the polar repul- 
sion manifested in the nuclear division which immediately follows 
fertilization, to the antagonism between male and female ele- 
ments. But, according to the important discoveries of Flemming 
and van Beneden, the polar repulsion in each nuclear division is 
not based on the antagonism between male and female loops, but 
depends upon the antagonism and mutual repulsion between the 
two halves of the same loop. The loops of the father and those 
of the mother remain together and divide together throughout 
the whole ontogeny. 

What can be the explanation of the fact that nuclear division 
follows immediately after fertilization, but that without fertilization 

* W. Boux, ' Ueber die Bedeutung der Kemtheilungsfiguren.' Leipzig, 1883. 



232 THE CONTINUITY OF THE GERM-PLASM AS THE 

it does not occur in most cases ? There is only one possible ex- 
planation, viz. the fact that the quantity of the nucleus has been 
suddenly doubled, as the result of conjugation. The difference 
between the male and female pronuclei cannot serve as an explana- 
tion, even though the nature of this difference is entirely unknown, 
because polar repulsion is not developed between the male and 
female halves of the nucleus, but within each male and each female 
half. We are thus forced to conclude that increase in the quantity 
of the nucleus affords an impulse for division, the disposition 
towards it being already present. It seems to me that this view 
does not encounter any theoretical difficulties, and that it is an 
entirely feasible hypothesis to suppose that, besides the internal 
conditions of the nucleus, its quantitative relation to the cell-body 
must be taken into especial account. It is imaginable, or perhaps 
even probable, that the nucleus enters upon division as soon as its 
idioplasm has attained a certain strength, quite apart from the 
supposition that certain internal conditions are necessary for this 
end. As above stated, such conditions may be present, but division 
may not occur because the right quantitative relation between 
nucleus and cell-body, or between the different kinds of nuclear 
idioplasm, has not been established. I imagine that such a quan- 
titative deficiency exists in an egg, which, after the expulsion of the 
ovogenetic nucleoplasm in the polar bodies, requires fertilization in 
order to begin segmentation. The fact that the polai' bodies were 
expelled proves that the quantity of the nucleus was sufficient to 
cause division, while afterwards it was no longer sufficient to pro- 
duce such a result. 

This suggestion will be made still clearer by an example. In 
Ascaris megalocepliala the nuclear substance of the female pro- 
nucleus forms two loops, and the male pronucleus does the same ; 
hence the segmentation nucleus contains four loops, and this is 
also the case with the first segmentation spheres. If we suppose 
that in embryonic development, the first nuclear division requires 
such an amount of nuclear substance as is necessary for the forma- 
tion of four loops, — it follows that an egg, which can only form 
two or three loops from its nuclear reticulum, would not be able to 
develope parthenogenetically, and that not even the first division 
would take place. If we further suppose that, while four loops 
are sufficient to start nuclear division, these loops must be of a 



FOUNDATION OF A THEORY OF HEREDITY. 233 

certain size and quantity in order to complete the whole ontogeny 
(in a certain species), it follows that egg-s possessing a reticulum 
which contains barely enough nuclear substance to divide into 
four segments, would be able to produce the first division and 
perhaps also the second and third, or some later division, but 
that at a certain point during ontogeny, the nuclear substance 
would become insufficient, and development would be arrested. 
This will occur in eggs which enter upon development without 
fertilization, but are arrested before its completion. One might 
compare this retardation leading to the final arrest of development, 
to a railway train which is intended to meet a number of other 
trains at various junctions, and which can only travel slowly 
because of some defect in the engine. It will be a little behind time 
at the first junction, but it may just catch the train, and it may 
also catch the second or even the third ; but it will be later at each 
successive junction, and will finally arrive too late for a certain 
train ; and after that it will miss all the trains at the remaining 
junctions. The nuclear substance grows continuously during de- 
velopment, but the rate at which it increases depends upon the 
nutritive conditions together with its initial quantity. The nu- 
tritive changes during the development of an egg depend upon 
the quantity of the cell-body which was present at the outset, and 
which cannot be increased. If the quantity of the nuclear sub- 
stance is rather too small at the beginning, it will become more and 
more insufficient in succeeding stages, as its growth becomes less 
vigorous, and difiers more from the standard it would have reached 
if the original quantity had been normal. Consequently it will 
gradually fall more and more short of the normal quantity, like 
the train which arrives later and later at each successive junction, 
because its engine, although with the full pressure of steam, is 
unable to attain the normal speed. 

It will be objected that four loops cannot be necessary for nuclear 
division in Ascaris, since such division takes place in the formation of 
the polar bodies, resulting in the appearance of the female pronucleus 
with only two loops. But this fact only shows that the quantity of 
nuclear substance necessary for the formation of four loops is not 
necessary for all nuclear divisions ; it does not disprove the assump- 
tion that such a quantity is required for the division of the seg- 
mentation nucleus. In addition to these considerations we must not 



234 THE CONTINUITY OF THE QEBM-PLASM AS THE 

leave the substance of the cell-body altogether out of account, for, 
although it is not the bearer of the tendencies of heredity, it must 
be necessary for every change undergone by the nucleus, and it 
surely also possesses the power of influencing changes to a large ex- 
tent. There must be some reason for the fact that in all animal 
eggs with which we are acquainted, the nucleus moves to the sur- 
face of the egg at the time of maturation, and there passes through 
its well-known transformation. It is obvious that it is there sub- 
jected to different influences from those which would have acted 
upon it in the centre of the cell-body, and it is clear that such an 
unequal cell-division as takes place in the separation of the polar 
bodies could not occur if the nucleus remained in the centre of 
the egg. 

This explanation of the necessity for fei-tilization does not exclude 
the possibility, that, under certain circumstances, the substance of 
the egg-nucleus may be larger, so that it is capable of forming four 
loops. Eggs which thus possess sufl[icient nucleoplasm, viz. germ- 
plasm," for the formation of the requisite four loops of normal size, 
(namely, of the size which would have been produced by fertilization), 
can and must develope by the parthenogenetic method. 

Of course the assumption that four loops must be formed has only 
been made for the sake of illustration. We do not yet know 
whether there are always exactly four loops in the segmentation 
nucleus^. I may add that, although the details by which these 
considerations are illustrated are based on arbitrary assumptions, the 
fundamental view that the development of the egg depends, ceteris 
paribus, upon the quantity of nuclear substance, is certainly right, 
and follows as a necessary conclusion from the ascertained facts. It 
is not unlikely that such a view may receive direct proof in the 
results of future investigations. Such proof might for instance be 
forthcoming if we were to ascertain, in the same species, the number 
of loops present in the segmentation nucleus of fertilization, as 
compared with those present in the segmentation nucleus of par- 
thenogenesis. 

The reproductive process in bees will perhaps be used as an argu- 
ment against my theory. In these insects, the same egg will de- 
velope into a female or male individual, according as fertilization 

* We now know that the number of loops vai-ies considerably in different species, 
even when they belong to the same gioup of animals (e.g. Nematodes). — A.W., 1888. 



FOUNDATION OF A THEORY OF HEREDITY. 235 

has or has not taken place, respectively. Hence, one and the same 
eg-g" is capable of fertilization, and al&o of parthenog-enetic develop- 
ment, if it does not receive a spermatozoon. It is in the power of 
the queen-bee to produce male or female individuals : by an act of 
will she decides whether the egg she is laying- is to be fertilized or 
unfertilized. She 'knows beforehand '^ whether an egg* will develope 
into a male or a female animal, and deposits the latter kind in the 
cells of queens and workers, the former in the cells of drones. It 
has been shown by the discoveries of Leuckart and von Siebold that 
all the eggs are capable of developing* into male individuals, and 
that they are only transformed into ' female eg'gs ' by fertilization. 
This fact seems to be incompatible with my theory as to the cause 
of parthenogenesis, for if the same egg", possessing exactly the same 
contents, and above all the same segmentation nucleus, may de- 
velope sexually or parthenogenetically, it appears that the power 
of parthenogenetic development must depend on some factor other 
than the quantity of germ-plasm. 

Although this appears to be the case, I believe that my theory 
encounters no real difficulty. I have no doubt whatever, that the 
same egg may develope with or without fertilization. From a care- 
ful study of the numerous ^cellent investigations upon this point 
which have been conducted in a particularly striking' manner by 
Bessels ^ (in addition to the observers quoted above), I have come 
to the conclusion that the fact is absolutely certain. It must be 
candidly admitted that the same egg will develope into a drone 
when not fertilized, or into a worker or queen when fertilized. One 
of Bessels' experiments is sufficient to prove this assertion. He 
cut off the wings of a young queen and thus rendered her incapable 
of taking ' the nuptial flight.' He then observed that all the eggs 
which she laid developed into male individuals. This experiment 
was made in order to prove that drones are produced by unfertilized 
eggs ; but it also proves that the assertion mentioned above is correct, 
for the eggs which ripen first and are therefore first laid, would have 

* This expression is used by bee-keepers, for instance by the well-known Baron 
Berlepsch. Of course, it would be more accurate to say that the queen, seeing the cell 
of a drone, is stimulated to lay an unfertilized egg, and that, on the other hand, she 
is stimulated to lay a fertilized egg when she sees the cell of a worker, or that of a 
queen. 

* E. Bessels, ' Die Landois'sche Theorie widerlegt durch das Experiment.' • 
Zeitschrift flir wissenschaftliche Zoologie, Bd. XVIII. p. 124. 1868. 



236 THE CONTINUITY OF THE GEEM-PLA8M AS THE 

been fertilized had the queen been impregnated. The supposition 
that, at certain times, the queen produces eg-g-s requiring- fertiliza- 
tion, while at other times her eggs develope parthenogenetically, is 
quite excluded by this experiment ; for it follows from it, that the 
eggs must all be of precisely the same kind, and that there is no 
difference between the eggs which require fertilization and those 
which do not. 

But does it therefore follow that the quantity of germ-plasm 
in the segmentation nucleus is not the factor which determines 
the beginning of embryonic development ? I believe not. It 
can be very well imagined that the nucleus of the egg, having 
expelled the ovogenetic nucleoplasm, may be increased to the 
size requisite for the segmentation nucleus in one of two ways : 
either by conjugation with a sperm-nucleus, or by simply growing 
to double its size. There is nothing improbable in this latter as- 
sumption, and one is even inclined to inquire why such growth 
does not take place in all unfertilized eggs. The true answer to 
this question must be that nature generally pursues the sexual 
method of reproduction, and that the only way in which the 
general occurrence of parthenogenesis could be prevented, was by 
the production of eggs which remained sterile unless they were 
fertilized. This was effected by a loss of the capability of growth 
on the part of the egg-nucleus after it had expelled the ovogenetic 
nucleoplasm. 

The case of the bee proves in a very striking manner that the 
difference between eggs which require fertilization, and those which 
do not, is not produced until after the maturation of the egg, and 
the removal of the ovogenetic nucleoplasm. The increase in the 
quantity of the germ-plasm cannot have taken place at any earlier 
period, or else the nucleus of the egg would always start embryonic 
development by itself, and the egg would probably be incapable of 
fertilization. For the relation between egg-nucleus and sperm- 
' nucleus is obviously based upon the fact that each of them is in- 
sufficient by itself, and requires completion. If such completion 
had taken place at an early stage the egg-nucleus would either 
cease to exercise any attractive force upon the sperm-nucleus, or 
else conjugation would be effected, as in Fol's interesting experi- 
ments upon fertilization by many spermatozoa ; and, as in these ex- 
periments, malformation of the embryo would result. In DajpAnidae 



FOUNDATION OF A THEORY OF HEREDITY. 237 

I believe I have shown ^ that the summer-eggs are not only de- 
veloped parthenogenetieally, but also that they are never fertil- 
ized ; and the explanation of this incapacity for fertilization may 
perhaps be found in the fact that their segmentation nucleus is 
already formed. 

We may therefore conclude that, in bees, the nucleus of the egg, 
formed during maturation, may either conjugate with the sperm- 
nucleus, or else if no spermatozoon reaches the egg may, under the 
stimulus of internal causes, grow to double its size, thus attaining 
the dimensions of the segmentation nucleus. For our present pur- 
pose we may leave out of consideration the fact that in the latter 
case the individual produced is a male, and in the former case a 
female. 

It is clear that such an increase in the germ-plasm must depend, 
to a certain extent, upon the nutrition of the nucleus, and thus in- 
directly upon the body of the egg-cell ; but the increase must chiefly 
depend upon internal nuclear conditions, viz. upon the capability of 
growth. We must further assume that the latter condition plays 
the chief part in the process, for everywhere in the organic world 
the limit of growth depends upon the internal conditions of the 
growing body, and can only be altered to a small extent by differ- 
ences of nutrition. The phyletic acquisition of the capability of 
parthenogenetic development must therefore depend upon an alter- 
ation in the capability of growth possessed by the nucleus of the 

egg- 

This theory of parthenogenesis most nearly approaches Stras- 
burger's views upon the subject, for he also explains the non-occur- 
rence of parthenogenetic development by the insufficient quantity 
of nucleoplasm remaining in the egg after the expulsion of polar 
bodies. The former theory differs however in that the occurrence 
of parthenogenesis is supposed to be only due to an increase of this 
nucleoplasm to the normal size of the seg-mentation nucleus. Stras- 
burger assumes that ' specially favourable conditions of nutrition 
counteract the deficiency of nuclear idioplasm,' while it seems to 
me that nutrition must be considered as only of secondary import- 
ance. Thus in bees, as above stated, the same egg may develope 
parthenogenetieally or after fertilization, the nucleus being subject 
to the same conditions of nutrition in both cases. Strasburger^ 
^ ' Daphniden,' Abhandlung, vi. p. 324. ^ I.e., p. 150. 



238 THE CONTINUITY OP THE GERM-PLASM AS THE 

considers that parthenogenesis may be interpreted by one of three 
possible explanations. First, he suggests that especially favourable 
nutrition may lead to the completion of the nuclear idioplasm. 
But if this assumption be made, we must ask why a part of the 
idioplasm should be previously expelled, when immediately after- 
wards the presence of an equal amount becomes necessary. Such a 
view can only be explained by the above-made assumption that the 
expelled nucleoplasm has a different constitution from that possessed 
by the nucleoplasm which is afterwards formed. It is true that we 
do not yet certainly know whether a polar body is expelled in eggs 
in which parthenogenesis occurs, but we do know that the egg of 
the bee passes through the same stages of maturation whether it 
is to be fertilized or not. I can hardly accept Strasburger's second 
suggestion, * that under some favourable conditions of nutrition half 
[or perhaps better, a quarter] of the idioplasm of the egg-nucleus 
is sufficient to start the processes of development in the cyto-idio- 
plasm.' Finally, his third suggestion, ' that the cyto-idioplasm, 
nourished by its surroundings and thus increased in quantity, com- 
pels the nucleus of the egg to enter upon division,' presupposes that 
the cell-body gives the impulse for nuclear division, a supposition 
which up to the present time remains at least unproved. The 
ascertained facts appear to me to indicate rather that the cell- 
body serves only as a medium for the nutrition of the nucleus, and 
Fol's recently mentioned observations, which have been especially 
quoted by Strasburger in support of his theories, seem to me to 
rather confirm my conclusions. If supernumerary sperm-nuclei 
penetrate into the egg, they may, under the nutritive influence of 
the cell-body, become centres of attraction, and may take the first 
step towards nuclear and cell-division by forming amphiasters. 
Such nuclei cannot control the whole cell-body and force it to 
divide, but each one of them, having grown to a certain size at the 
expense of the cell-body, makes its influence felt over a certain area. 
Strasburger is quite right in considering this process as a ' partial 
parthenogenesis.' Such partial parthenogenesis presumably occurs 
in all egg-nuclei, but the latter cannot attain to complete partheno- 
genesis when, as in Fol's supernumerary sperm-nuclei, their powers 
of assimilation are insufficient to enable them to reach the requisite 
size. As before stated, the cell-body does not force the nucleus to 
divide, but vice versa. It would, moreover, be quite erroneous to 



FOUNDATION OF A THEORY OF HEREDITY. 239 

suppose that parthenog-enetic eg-g-s must contain a larg-er amount of 
nutritive material in order to facilitate the growth of the nucleus. 
The parthenogenetic eggs of certain Baphnidae [Bythotrephes^ Poly- 
phemus) are very much smaller than the winter-eggs, which require 
fertilization, in the same species. It is also an error for Strasburger 
to conclude that ' it has been established with certainty that favour- 
able conditions of nutrition cause parthenogenetic development in 
Baphnidae, while unfavourable conditions cause the formation of 
eggs requiring fertilization.' It is true that Carl Diising^, in his 
notable work upon the origin of sex, has attempted, in a most 
ingenious manner, to prove, from my observations and experiments 
on the reproduction of Baphnidae, ' that winter or summer-eggs are 
formed according to the nutritive condition of the ovary.' I do 
not, however, believe that he has succeeded in this attempt, and 
at all events it is quite clear that the validity of such conclusions 
is not fully established. I have observed that the maturing eggs 
break up in the ovaries and are absorbed in those Baphnidae 
(Sida) which are starved because sufficient food cannot be pro- 
vided in captivity. Hence such animals live, as it were, at the 
expense of their descendants ; but it would be quite erroneous 
to conclude with Diising, from the similarity which such disap- 
pearing egg-follicles bear to the groups of germ-cells which 
normally break up in the formation of winter-eggs, that with 
a less degree of starvation winter-eggs would have been formed. 
Diising further quotes my incidental remark that the formation of 
resting-eggs in Baphnla has been especially frequent in aquaria 
' which had been for some time neglected, and in which it was 
found that a great increase in the number of individuals had 
taken place.' He is entirely wrong in concluding that there 
was any want of food in these neglected aquaria ; and if I had 
foreseen that such conclusions would have been drawn, I might 
have easily guarded against them by adding that in these very 
aquaria an undisturbed growth of different algae was flourishing, 
so that there could have been no deficiency, but, on the contrary, 
a great abundance of nutritive material. I may add that since 
that time I have conducted some experiments directly bearing upon 
this question, by bringing virgin females as near to the verge of 

' Carl Diising, ' Die Regulirung des GescHechtsverhaltnisses.' .Tena, 1884. 



240 THE CONTINUITY OF THE GERM-PLASM AS THE 

starvation as possible, but in no case did they enter upon sexual 
reproduction ^. 

An author must have been to some extent misled by preconceived 
ideas when he is unable to see that the manner in which the two 
kinds of eggs are respectively formed, directly excludes the possi- 
bility of the origin of sexual eggs from the effects of deficient or 
poor nutrition. The resting eggs, which require fertilization, are 
always larger, and require for their formation far more nutritive 
material, than the parthenogenetic summer-eggs. In 3IoifUz, for 
instance, forty large food-cells are necessary for the formation of 
a resting egg, while a summer-egg only requires three. And 
Diising is aware of these facts, and quotes them. How can the 
formation of resting eggs depend upon the effects of poor nutrition 
when food is most abundant at the very time of their formation ? 
In all those species which inhabit lakes, sexual reproduction occurs 
towards the autumn, and in such cases the resting eggs are true 
winter-eggs, destined to preserve the species during the winter. 
But at no time of the year is the food of the Daphnidae so abundant 
as in September and October, and frequently even until late in 
November (in South Germany). At this period of the year, the 
water is filled with flakes of animal and vegetable matter in a state 
of partial decomposition, thus affording abundant food for many 
species. It also swarms with a large number of species of Crustacea, 
Radiolaria, and Infusoria ; and thus such Daphnids as the Poly- 
pkemidae are also well provided for. Hence there is no deficiency 
in the supply of food. Any one who has used a fine net in our fresh 
waters at this time of the year must have been at first astonished 
at the enonnous abundance of the lower forms of animal life ; and 
he must have been much more astonished if he has been able to 
compare such results with the scanty population of the same 
localities in spring. But it is during the spring and summer that 
these very Daphnidae reproduce themselves parthenogenetically. 
I am far from believing that my experiments on Daphnidae are 
exhaustive and final, and I have stated this in my published 
writings on the subject ; but it seems to me that I have established 
the fact that direct influences, whether of food or of temperature, 
acting upon single individuals, do not determine the kind of eggs 

^ I intend to publish these experiments elsewhere in connexion with other 
observations. 



FOUNDATION OF A THEORY OF HEREDITY. 241 

which are to be produced ; but that such a decisive influence is to be 
found in the indirect conditions of life, and especially in the 
average frequency of the recurrence of adverse circumstances which 
kill whole colonies at once, such as the winter cold, or the drying- 
up of small ponds in summer. It is unnecessary for me to contro- 
vert Diising in detail, as I have already taken this course in the 
case of Herbert Spencer ^, who had also formed the hypothesis that 
diminished nutrition causes sexual reproduction. 

One of my observations seems, indeed, to support such a view, but 
only when it is considered as an isolated example. I refer to the 
behaviour of the genus Moina. Females of this genus which 
possess sexual eggs in their ovaries, and which would have con- 
tinued to produce such eggs if males had been present, enter in 
the absence of the latter upon the formation of parthenogenetic 
summer-eggs, that is, if the sexual eggs have not all been extruded, 
but have been re-absorbed in the ovary. At first sight, indeed, such 
a result appears to indicate that the increase in nutrition, produced 
by the breaking-up of the large winter-egg in the ovary, deter- 
mines the formation of parthenogenetic eggs. This apparent con- 
clusion seems to be fui'ther confirmed by the following fact. The 
transition from sexual to parthenogenetic reproduction only occurs 
in one species of Moina [M. rectirostris), but in this species it occurs 
always and without exception, while in the other species which I 
have investigated i^I. jparadoxd), winter-eggs, when once formed, are 
always laid, and such females can never produce summer-eggs. 
But in sj)ite of this fact, Diising is mistaken when he explains the 
continuous formation of sexual eggs in the latter species as due \o 
the absence of any great increase in the amount of nutrition, such 
as would have followed if the ^^^ had broken up in the ovary. 
In many other Baphnidae which have come under my notice, the 
females frequently enter again upon the formation of partheno- 
genetic summer-eggs, after having laid fertilized resting eggs, 
upon one or more occasions. This is the case, for instance, in all 
the species of Uaphnia with which I am acquainted, and such 
a fact at once proves that the abnormal increase in nutrition 
produced by the absorption of winter-eggs cannot be the cause of 
the succeeding parthenogenesis. It also supports the proof that 

' Weismann, ' Daphniden,' Abhandlung, VII. p. 329; Herbert Spencer, 'The 
Principles of Biology,' 1864, vol. i. pp. 229, 230. 

R 



242 THE CONTINUITY OF THE QEBM-PLASM AS THE 

a hig-h or low nutritive condition of the whole animal can have 
nothing" to do with the kind of eggs which are produced, for in 
the above-quoted instance, the nutrition has remained the same 
througfhout, or at all events has not been increased. It is erroneous 
to always look for the explanation of the mode of eg-g'-formation in 
the direct action of external causes. Of course there must be 
direct causes which determine that one germ shall become a winter- 
eg-g-, and another a summer-egg; but such causes do not lie outside 
the animal, and have nothing to do with the nutritive condition of 
the ovary : they are to be found in those conditions which we are 
not at present able to analyze further, and which we must, in the 
meantime, call the specific constitution of the species. In the young 
males of Dap/midae the testes have precisely the same appearance 
as the ovaries of the young females ■', but the former will, never- 
theless, produce sperm-cells and not ova. In such cases the sex of 
the young individual can always be identified by the form of the 
first antenna and of the first thoracic appendage, both of which 
are always clawed in the male. But who can point to the direct 
causes which determine that the sexual cells shall become sperm- 
cells in this case, and not egg-cells ? Does the determining cause 
depend on the conditions of nutrition ? Or, again, in the females, 
can the state of nutrition determine that the third out of a group 
of four germ-cells shall become an egg-cell, and that the others 
shall break up to serve as its food ? 

It is, I think, clear that these are obvious instances of the general 
conclusion that the direct causes determining the direction of 
development in each case are not to be looked for in external con- 
ditions, but in the constitution of the organs concerned. 

We arrive at a like conclusion when we consider the quality of 
the eggs which are produced. The constitution of one species of 
Moina contains the cauSe which determines that each indiWdual 
shall produce winter-eggs only, or summer-eggs only; while in 
another species the transition from the formation of sexual eggs to 
the formation of summer-eggs can take place, but only when the 
winter-egg remains unfertilized. The latter case appears to me to 
be notably a special adaptation, in this and other species, to the 
deficiency of males, which is apt to occur. At all events, it is 

• The same fact has since been ascertained in specie'^ belonging to several groups 
of animals. 



FOUNDATION OF A THEORY OF HEREDITY. 243 

obvious that it is an advantage that an unfertilized sexual egg 
shall not be lost to the organism. The re-absorption of the winter- 
egg is an arrangement which, without being the cause, is favourable 
to the production of summer-eggs. 

This subject is by no means a simple one, as is proved by the 
behaviour of the small group of Dap/midae. Thus in some species, 
the winter-eggs are produced by purely sexual females, which never 
enter upon parthenogenesis ; in others, the sexual females may take 
the latter course, but only when males are absent ; in others, again, 
they regularly enter upon parthenogenesis. In my work on 
Daphiidae, I have attempted to show that their behaviour in this 
respect is associated with the various external conditions under 
which the ■ different species live ; and also that the ultimate 
occurrence of the sexual period, and finally the whole cyclical 
alternation of sexual and parthenogenetic reproduction, depend 
upon adaptation to certain external conditions of life. 

With the aid of my hypothesis that the egg-nucleus is com- 
posed of ovogenetic nucleoplasm and germ-plasm, I can now 
attempt to give an approximate explanation of the nature and 
origin of the direct causes which determine the production, at one 
time of parthenogenetic summer-eggs, and at another time of 
winter-eggs, requiring fertilization. But in such an explanation I 
should also wish to include a consideration of the causes which de- 
termine the formation of the nutritive cells of the egg and of the 
sperm-cells to which I have alluded above. 

I believe that the direct cause which determines why the 
apparently identical cells of the young testis and ovary in the 
Dap/midae develope in such different directions, is to be found in the 
fact, that their nuclei possess different histogenetic nucleoplasms, 
while, if we neglect individual differences, the germ-plasm remains 
precisely the same. In the sperm-cells the histogenetic nucleoplasm 
is spermogenetic, in the egg-cells it is ovogenetic. This must be 
conceded if our fundamental view is correct, that the specific nature 
of the cell-body is determined by the nature of its nucleus. 

Similarly, the germ-cells of female Uap/midae, which at first do 
not exhibit the smallest differences, must really differ in that their 
nuclei must contain different kinds of nucleoplasm, which are 
present in different proportions. Germ-cells which are to produce 
a finely granular, brick-red, winter yolk [Moiua rectirostris) must 

R 2 



244 THE CONTINUITY OF THE GERM-PLASM AS THE 

possess an ovogenetic nucleoplasm of a somewhat different mole- 
cular structure from those germ-cells which have only to form 
a few large blue fat-globules, as in the summer-eggs of the same 
species. It is further probable that different proportions obtain be- 
tween germ-plasm and ovogenetic nucleoplasm, in these two kinds 
of germ-cells ; and it would be a very simple explanation of the 
otherwise obscure part played by the food-cells, if we were to 
suppose that they do not contain any germ-plasm at all, and on 
this account do not enter upon embryonic development, but are 
arrested after growing to a certain size. Such an explanation, 
however, would not by itself show why they subsequently undergo 
gradual solution in the surrounding fluids. But since we know 
that egg-cells also begin to undergo solution as soon as the parent 
Daphnid is poorly nourished, we can hardly help also referring the 
solution of the food-cells to insufficient nourishment, occurring as 
soon as the egg-cell, after the attainment of a certain size, exercises 
a superior power of assimilation. But hitherto we could not in any 
way understand why the third out of a group of germ-cells should 
always gain this superior power and become an egg-cell. If it 
could be shown that its position is more highly favoured in respect 
of nutrition, we could understand why it outstrips the other three 
in development, and thus prevents them from further growth. 
But nothing of the kind can be shown to occur with any degree of 
probability, as I have previously mentioned in my works on the 
subject. At that time, having no better explanation, I adopted 
the view in question, although only as a provisional intei*preta- 
tion. It was not possible for me to seek in the substance of 
those four apparently identical cells for the cause of their different 
development ; but now I am justified in offering the supposition 
that during the division of a primitive germ-cell into two, and after- 
wards into four germ-cells, an unequal division of the nucleoplasms 
takes place, in that one of the four cells receives germ-plasm as 
well as ovogenetic nucleoplasm, while the other three receive the 
latter alone. Similarly, the fact that the second cell of the group 
may occasionally become an egg is also intelligible, although this 
fact remained quite inexplicable by my former interpretation. The 
fact that true egg-cells, or even the whole ovary with all its germ- 
cells, may break up and become absorbed when the animal has been 
starved for a certain period of time, seems to me to be no objection 



FOUNDATION OF A THEORY OF HEREDITY. 245 

to our present view, any more than the fact that an Infusorian may 
die from starvation would be an objection to the supposition of the 
immortality of unicellular organisms. The growth of an organism 
is not only arrested by its constitution, but also by absolute want 
of food ; but it would be very foolish to explain the differences 
in size of the various species of animals as results of the different 
conditions of nutrition to which they were subject. Just as 
a sparrow, however highly nourished, could never attain the size or 
form of an eagle, so a germ-cell destined to become a summer-egg 
could never attain the size, form, or colour of a winter-egg. It is 
by internal constitutional causes that the course of development is 
determined in both these cases ; and in the latter, the cause can 
hardly be anything more than the different constitution of the 
nucleoplasms. 

All these considerations depend upon the supposition that the 
egg-nucleus contains two kinds of idioplasm, viz. germ-plasm and 
ovogenetic nucleoplasm. I have not hitherto brought forward any 
direct evidence in favour of this assumption, but I believe that such 
proofs can be obtained. 

It is well known that there are certain eggs in which the polar 
bodies are not expelled until after the entrance of spermatozoa. 
Brooks^ has already made use of this fact as evidence against 
Minot's and Balfour's theory ; for he quite rightly concludes that 
if the polar bodies really possess the significance of male cells, we 
cannot understand why such eggs are unable to develope without 
fertilization, when they still possess the male half of the nucleus 
necessary for development. But such eggs (e.g. that of the oyster) 
do not develope, but always die if they remain unfertilized. 

This argument can only be met by a new hypothesis, the con- 
struction of which I must leave to the defenders of the above- 
mentioned theory. But the observation in question seems to me 
to furnish at the same time a proof of the co-existence of two 
different nucleoplasms in the egg-nucleus. If the nucleoplasm of 
the polar bodies was also germ-plasm, we could not understand 
why such eggs are unable to develope parthenogenetically, for at 
least as much germ-plasm is contained in the unfertilized egg as 
would have been present after fertilization. 

* Brooks, 'The Law of Heredity.' Baltimore, 1883, p. 73. 



246 THE CONTINUITY OF THE GERM-PLASM AS THE 

The only objection which can be raised against this conclusion 
depends upon the supposition that the nucleoplasm of the sperm- 
cell is qualitatively different from that of the egg-cell. I have 
already dealt with this view, but I should wish to refer to it again 
rather more in detail. Some years ago I expressed the opinion ^ 
that the physiological values of the sperm-cell and of the egg-cell 
must be identical ; that they stand in the ratio of i : i. But 
Valaoritis ^ has brought forward the objection that if we consider 
the function of a cell as the measure of its physiological value, it is 
only necessary to point to the respective functions of ovum and 
spermatozoon in order to show that their physiological values must 
be different. ' The egg-cell alone, by passing more or less com- 
pletely through the phyletic stages of the female parent, developes 
into a similar organism ; and although the presence of the sperma- 
tozoon is in most cases required in order to render possible such a 
result, the cases of parthenogenesis prove nevertheless that the 
egg can do without this stimulus.' This objection appeared to be 
fully justified as long as fertilization was looked upon as the ' vital- 
ization of the germ,' and so long as the sperm-cell was considered 
as merely 'the spark that kindles the gunpowder,' and further 
so long as the germ-substance was believed to be contained in the 
cell-body. But now we can hardly give to the body of the egg- 
cell a higher significance than that of the common nutritive 
soil of the two nuclei which conjugate in fertilization. But 
these two nuclei ' are not different in nature,' as Strasburger says, 
and as I fully believe. They cannot differ in kind, for they both 
consist of germ-plasm belonging to the same species of animal or 
plant ; and there cannot be any deeper contrast between them such 
as would con-espond to the differences between mature individuals. 
They cannot, from their essential nature, exercise any special at- 
traction upon each other, and when we see that sperm-cell and egg- 
cell do nevertheless attract each other, as has been shown in both 
plants and animals, such a property must have been secondarily 
acquired, and has no other significance than to favour the union of 
sexual cells — an arrangement which may be compared to the vi- 
brating flagellum of the spermatozoon or the micropyle of the egg, 
but which is not fundamental, and is not based upon the molecular 

' ' Zeitschrift fiir wissenschaftliche Zoologie,' Bd. XXXIII. p. 107. 1S73, 
' Valaoritis, 1. c, p. 6. 



FOUNDATION OP A THEORY OF HEREDITY. 247 

structure of the g-erm-plasm. In lower plants, PfefFer has proved 
that certain chemical stimuli emanate from the eg-g and attract the 
sperm atozoid ; and according to Strasburger, the synergidae in the 
upper part of the embryo-sac of Phanerogams secrete a substance 
which is capable of directing the growth of the pollen-tube towards 
the egg-cell. In animals it is only known as yet that spermatozoa 
and ova do attract each other, so that the former find the latter and 
bore their way through its membranes. It has also been shown 
that the substance of the egg-body moves towards the pene- 
trating spermatozoon {^ cones d' exstidaiion in Asteridae : Fol) ; and 
that it sometimes enters upon convulsive movements [Petromi/zon). 
Here therefore a mutual stimulation and attraction must exist ; 
and perhaps we must also assume that there is an attraction be- 
tween the two conjugating nuclei, for we cannot readily understand 
how the cytoplasm alone could direct the one to the other, as 
Strasburger supposes. According to Strasburger's hypothesis, we 
must suppose that part of the specific cytoplasm of the sj)erm-cell 
continues to surround the nucleus after it has penetrated into the 
body of the egg. But however this may be, the assumed attraction 
between the conjugating nuclei certainly cannot depend upon the 
molecular structure of their germ-plasm, which is the same in both, 
but it must be due to some accessory circumstance. If it were 
possible to introduce the female pronucleus of an egg into another 
egg of the same species, immediately after the transformation of the 
nucleus of the latter into the female pronucleus, it is very probable 
that the two nuclei would conjugate just as if a fertilizing sperm- 
nucleus had penetrated. If this were so, the direct proof that egg- 
nucleus and sperm-nucleus are identical would be furnished. Un- 
fortunately the practical difficulties are so great that it is hardly 
possible that the experiment can ever be made ; but such want of 
experimental proof is partially compensated for by the fact, ascer- 
tained by Berthold, that in certain Algae [Ectocarpus and Sci/tosi- 
pJion) there is not only a female, but also a male parthenogenesis ; for 
he shows that in these species the male germ-cells may sometimes 
develope into plants, which however are very weakly ^. Furthermore 

* I quote from Falkenberg, in Schenk's 'Handbuch der Botanik,' Bd. II. p. 219. 
He further states that these are the only instances hitherto known in which un- 
doubted male cells have proved to be capable of further development when they have 
been unable to exercise their powers of fertilization. It must be added that the two 
kinds of germ-cells do not differ in appearance, but only in behaviour ; the female 



248 THE CONTINUITY OP THE GEBM-PLA8M, ETC. 

•the process of conjugation may be considered as a proof that this 
view as to the secondary importance of sexual differentiation is 
the true one. At the present time there can hardly be any hesita- 
tion in accepting the view that conjugation is the sexual repro- 
duction of unicellular organisms. In these the two conjugating 
cells are almost always identical in appearance, and there is no 
evidence in favour of the assumption that they are not also identical 
in molecular structure, at least so fer as one individual of the 
same species may be identical with another. But there are also 
forms in which the conjugating cells are distinctly differentiated 
into male and female, and these are connected with the former by 
a gradual transition : thus in Fandor'ina, a genus of Volvocineae, we 
are unable to make out any differences between the conjugating 
cells, while large egg-cells and minute sperm-cells exist in the 
closely allied Volvox. If we must suppose that the conjugation of 
two entirely identical Infusoria has the same physiological effect as 
the union of two sexual cells in higher animals and plants, we can- 
not escape this conclusion that the process is essentially the same 
throughout : and that therefore the differences, which are perhaps 
already indicated in Paridorina and are very distinct in Volvox and 
in all higher organisms, have nothing to do with the nature 
of the process, but are of quite secondary importance. If we further 
take into account the extremely different constitution of the two 
kinds of sexual cells in size, appearance, membranes, motile power, 
and finally in number, no doubt remains that these differences are 
only adaptations which secure the meeting of the two kinds of 
conjugating cells : that in each species they are adaptations to the 
peculiar conditions under which fertilization takes place. 

germ-cells becoming fixed, and withdrawing one of their two flagella, while the male 
cells continue to swarm. But even this slight degree of differentiation requires the 
supposition of internal molecular differentiation. 



t9i 



NOTE. 

It is of considerable importance for the proper appreciation of 
the views advanced in the present essay, to ascertain whether a 
polar body is or is not expelled from eggs which develope partheno- 
genetically. I wish therefore to briefly state that I have recently 
succeeded in proving- the formation of a polar body of distinctly 
cellular structure in the summer-eg-gs of I)a/p1inidae. I propose to 
publish a more detailed account in a future paper. 

A. W. 

June 2 2, 1885. 



THE SIGNIFICANCE OF SEXUAL EEPEODUCTION 
IN THE THEORY OF NATUEAL SELECTION. 

1886. 



SIGNIFICANCE OF SEXUAL REPRODUCTION, etc. 



PREFACE. 

The greater part of the present essay was delivered at the first 
general meeting of the Association of German Naturalists, at 
Strassburg, on September i8th, 1885, and is printed in the Pro- 
ceedings of the fifty-eighth meeting of that Society. 

The form of a lecture has been retained in the present publican 
tion, but its contents have been extended in many ways. Besides 
many small and a few large additions to the text, I have added 
six appendices in order to treat of certain subjects more fully than 
was possible in the lecture itself, in which I was often obliged to be 
content with mere hints and suggestions. This appears to be all 
the more necessary because it is impossible to suppose that many 
views and ideas upon which the lecture was based would be well 
known to all readers, although they have been described in my 
former papers. It was above all necessary to deal with the class of 
acquired characters, which, as it seems to me, is easily confounded, 
especially by the medical profession, with the much broader class of 
new characters generally. Only those new characters can be called 
'acquired' which owe their origin to external influences, and the 
term 'acquired' must be denied to those which depend upon the 
mysterious relationship between the different hereditary tendencies 
which meet in the fertilized ovum. These latter are not ' acquired ' 
but inherited, although the ancestors did not possess them as such, 
but only as it were the elements of which they are composed. 
Such new characters as these do not at present admit of an exact 
analysis : we have to be satisfied with the undoubted fact of their 
occurrence. The transmission or non-transmission of acquired cha- 
racters must be of the highest importance for a theory of heredity, 
and therefore for the true appreciation of the causes which lead 
to the transformation of species. Any one who believes, as I do, 
that acquired characters are not transmitted, will be compelled to 



SIGNIFICANCE OF SEXUAL KEPRODUCTION, ETC. — PREFACE. 253 

assume that the process of natural selection has had a far larger 
share in the transformation of species than has been as yet 
accorded to it ; for if such characters are not transmitted, the 
modifying influence of external circumstances in many cases re- 
mains restricted to the individual, and cannot have any part in 
producing transformation. We shall also be compelled to abandon 
the ideas as to the origin of individual variability which have 
been hitherto accepted, and shall be obliged to look for a new 
source of this phenomenon, upon which the processes of selection 
entirely depend. 

In the following pages I have attempted to suggest such a 
source. 

A. W. 
Freiburg i. Br., 

November 22, 1885. 



SIGNIFICANCE OF SEXUAL REPRODUCTION, etc. 



CONTENTS. 

PAOB 

1. Can we dispense with the principle of natural selection ? .... 255 

2. Nageli's theory of transformation from internal causes . . . 256 

3. A definite course of development is possible without a self-changing idio- 

plasm 258 

4. Conclusive importance of * adaptations ' 260 

5. The structure of whales as an example of adaptation .... 261 

6. Transformation takes place by the smallest steps ..... 264 

7. The foundation of such minute changes depends upon individual variability 266 

8. Difficulty in accounting for variability on the supposition of a continuity of 

the germ-plasm ........... 266 

9. Previous theories by which variability has been accounted for . . . 267 

10. Non-transmission of acquired characters ....... 267 

11. Nageli's and Alexis Jordan's experiments ....... 269 

12. Germ-plasm is only altered with great difficulty . . . . -271 

13. The source of individual variation lies in sexual reproduction . . . 272 

14. The process of natural selection does not operate when asexual repro- 

duction takes place . . . . . . . . . -274 

15. Origin of variability in unicellular organisms 278 

16. Sexual reproduction effects combination 279 

17. E. van Beneden's and V. Hensen's theory of sexual reproduction as a process 

of rejuvenescence ........... 282 

18. Theoretical objections to such a view 2S3 

19. Original significance of conjugation 286 

20. Preservation of sexual reproduction by means of heredity . . . .287 

21. It is lost in parthenogenesis for reasons of utility 289 

22. Parthenogenesis prevents further transformations ..... 290 

23. It excludes Panmixia and thus prevents disused organs from becoming 

rudimentary 291 

24. Final considerations 294 



APPENDICES. 

I. FOBTHER CONSIDERATIONS WHICH OPPOSE NaGELI's EXPLANATION OP 

Transformation as due to internal causes 298 

II. Nageli's Explanation of Adaptation 300 

III. Adaptations in Plants 308 

IV. On the Supposed Transmission of Acquired Characters . . .310 
1. Brown-St^quard's experiments on Guinea-pigs 310 

'Z. A case which at first sight appears to prove the transmission of acquired 

characters 320 

V. On the Origin of Parthenogenesis 323 

VI. W. K. Brooks' Theory of Hereditv 326 



V. 

THE SIGNIFICANCE OF SEXUAL REPRODUCTION IN 
THE THEORY OF NATURAL SELECTION. 

During the quarter of a century whieli has elapsed since Biology 
began to occupy itself again with general problems, at least one 
main fact has been made clear by the united labours of numerous 
men of science, viz. the fact that the Theory of Descent, the idea 
of development in the organic world, is the only conception as to 
the origin of the latter, which is scientifically tenable. It is not 
only that, in the light of this theory, numerous facts receive for the 
first time a meaning and significance ; it is not only that, under 
its influence, all the ascertained facts can be harmoniously grouped 
together ; but in some departments it has already yielded the 
highest results which can be expected from any theory, it has 
rendered possible the prediction of facts, not indeed with the abso- 
lute certainty of calculation, but still with a high degree of 
probability. It has been predicted that man, who, in the adult 
state, only possesses twelve pairs of ribs, would be found to have 
thirteen or fourteen in the embryonic state : it has been predicted 
that, at this early period in his existence, he would possess the 
insignificant remnant of a very small bone in the wrist, the so- 
called OS centi-ale, which must have existed in the adult condition 
of his extremely remote ancestors. Both predictions have been 
fulfilled, just as the planet Neptune was discovered after its ex- 
istence had been predicted from the disturbances induced in the 
orbit of Uranus. 

That existing species have not arisen independently, but have 
been derived from other and mostly extinct species, and that on 
the whole this development has taken place in the direction of 
greater complexity, may be maintained with the same degree of 
certainty as that with which astronomy asserts that the earth 
moves round the sun ; for a conclusion may be arrived at as safely 
by other methods as by mathematical calculation. 

If I make this assertion so unhesitatingly, I do not make it in 
the belief that I am bring-ing" forward anything new nor because 



256 THE SIGNIFICANCE OF SEXUAL BEPRODUCTION 

I think that any opposition will be encountered, but simply because 
I wish to begin by pointing out the firm ground on which we 
stand, before considering the numerous problems which still remain 
unsolved. Such problems appear as soon as we pass from the facts 
of the case to their explanation ; as soon as we pass from the state- 
ment ' The organic world has arisen by development,' to the ques- 
tion ' But how has this been efiected, by the action of what forces, 
by what means, and under what circumstances ?' 

In attempting to answer these questions we are very far fi-om 
dealing with certainties ; and opinions are still conflicting. But 
the answer lies in the domain of future investigation, that un- 
known country which we have to explore. 

It is true that this country is not entirely unknown, and if I am 
not mistaken, Charles Darwin, who in our time has been the first 
to revive the long-dormant theory of descent, has already given a 
sketch, which may well serve as a basis for the complete map of the 
domain ; although perhaps many details will be added, and many 
others taken away. . In the principle of natural selection, Darwin 
has indicated the route by which we must enter this unkno^vn land. 

But this opinion is not universal, and only recently Carl Niigeli ^, 
the famous botanist, has expressed decided doubts as to the general 
applicability of the principle of natural selection. According to 
Niigeli, the co-operation of the external conditions of life with the 
known forces of the organism, viz. heredity and variability, are in- 
sufficient to explain the regular course of development pursued by 
the organic world. He considers that natural selection is at best 
an auxiliary principle, which accepts or rejects existing characters, 
' but which is unable to create anything new : he believes that the 
causes of transformation reside within the organism alone. Nageli 
further assumes that organisms contain forces which cause period- 
ical transformation of the species, and he imagines that the organic 
world, as a whole, has arisen in a manner similar to that in which 
a single individual arises. 

Just as a seed produces a certain plant because it possesses a 
certain constitution, and just as, in this process, certain conditions 
must be favourable (light, warmth, moisture, &c.) in order that 
development may take place, although they do not determine the 

1 C. Nageli, ' Mechanisch-physiologische Theorie der Abstammungslehre.' Munchen 
u. Leipzig, 1884. 



IN THE THEOEY OF NATURAL SELECTION. 257 

kiiid or the manner of development ; so, in precisely the same way, 
the tree of the whole organic world has grown up from the first 
and lowest forms of life on our planet, under a necessity arising 
from within, and on the whole independently of external influences. 
According to Nageli, the cause which compels every form of living 
substance to change, from time to time, in the course of its secular 
growth, and which moulds it afresh into new species, must lie 
within the organic substance itself, and must depend upon its mole- 
cular structure. 

It is with sincere admiration and real pleasure that we read the 
exposition in which Nageli gives, as it were, the result of all his 
researches which bear upon the great question of the development 
of the organic world. But although we derive true enjoyment from 
the contemplation of the elaborate and ingeniously wrought-out 
theoretical conception, — which like a beautiful building or a work of 
art is complete in itself, — and although we must be convinced that 
its rise has depended upon the progress of knowledge, and that by 
its means we shall eventually reach a fuller knowledge ; it is never- 
theless true that we cannot accept the author's fundamental 
hypothesis. I at least believe that I am not alone in this respect, 
and that but few zoologists will be found who can adopt the hypo- 
thesis which forms the foundation of Nageli's theory. 

It is not my intention at present to justify my own widely 
different views, but the subject of this lecture compels me to briefly 
explain my position in relation to Nageli, and to give some of the 
reasons why I cannot accept his theory of an active force of trans-^ 
formation arising and working within the organism ; and I must 
also explain the reasons which induce me to adhere to the theory of 
natural selection. 

The supposition of such a phyletic force of transformation (see 
Appendix I, p. 298) possesses, in my opinion, the greatest defect that 
any theory can have, — it does not explain the phenomena. I do not 
mean to imply that it is incapable of rendering certain subordinate 
phenomena intelligible, but that it leaves a larger number of facts 
entirely unexplained. It does not aflbrd any explanation of the 
purposefulness seen in organisms : and this is just the main problem 
which the organic world offers for our solution. That species are, 
from time to time, transformed into new ones might perhaps be 
understood by means of an internal transforming force, but that 

S 



258 THE SIGNIFICANCE OP SEXUAL REPRODUCTION 

they are so changed as to become better adapted to the new con- 
ditions under which they have to live, is left entirely unintellig-ible 
by this theory. For we certainly cannot accept as an explanation 
Nageli's statement that org-anisms possess the power of being- 
transformed in an adaptive manner simply by the action of an 
external stimulus (see Appendix II, p. 300). 

In addition to this fundamental defect, we must also note that 
there are absolutely no proofs in support of the foundation of this 
theory, viz. of the existence of an internal transforming force. 

Niigeli has very ingeniously worked out his conception of idio- 
plasm, and this conception is certainly an important acquisition 
and one that will last, although without the special meaning given 
to it by its author. But is this special meaning anything more than 
pure hypothesis ? Can we say more than this of the ingenious de- 
scription of the minute molecular structure of the hypothetical 
basis of life ? Could not idioplasm be built up in a manner entirely 
different from that which Nageli supposes ? And can conclusions 
drawn from its supposed structure be brought forward to prove 
anything ? The only proof that idioplasm must necessarily change, 
in the course of time, as the result of its own structure, is to be 
found in the fact that Nageli has so constructed it ; and no one 
will doubt that the structure of idioplasm might have been so con- 
ceived as to render any transformation from within itself entirely 
impossible. 

But even if it is theoretically possible to imagine that idioplasm 
possesses such a structure that it changes in a certain manner, as 
the result of mere growth, we should not be justified in thus 
assuming the existence of a new and totally unkno\vn principle 
until it had been proved that known forces are insufficient for the 
explanation of the observed phenomena. 

Can any one assert that this proof has been forthcoming ? It 
has been again and again pointed out that the phyletic development 
of the vegetable kingdom proceeds with regularit}'^ and according to 
law, as we see in the preponderance and constancy of so-called 
purely ' morphological ' characters in plants. The formation of 
natural groups in the animal and vegetable kingdoms comj^ls us 
to admit that organic evolution has frequently proceeded i'or longer 
or shorter periods along certain developmental lines. But we are 
not on this account compelled to adopt the supposition of un- 



IN THE THEORY OF NATURAL SELECTION. 259 

known internal forces which have determined such lines of de- 
velopment. 

Many years ago I attempted to prove ^ that the constitution or 
physical nature of an organism must exercise a restricting in- 
fluence upon its capacity for variation. A given species cannot 
change into any other species, which may be thought of. A beetle 
could not be transformed into a vertebrate animal : it could not 
even become a grasshopper or a butterfly ; but it could change into 
a new species of beetle, although only at first into a species of 
the same genus. Every new species must have been directly con- 
tinuous with the old one from which it arose, and this fact alone 
implies that phyletic development must necessarily follow certain 
lines. 

I can fully understand how it is that a botanist has more incli- 
nation than a zoologist to take refuge in internal developmental 
forces. The relation of form to function, the adaptation of the 
organism to the internal and external conditions of life, is less 
prominent in plants than in animals : and it is even true that 
a large amount of observation and ingenuity is often necessary in 
order to make out any adaptation at all. The temptation to 
accept the view that everything depends upon internal directing 
causes is therefore all the greater. Nageli indeed looks at the 
subject from the opjiosite point of view, and considers that the 
true underlying cause of transformation is in animals obscured by 
adaptation, but is more apparent in plants ^. Sufiicient justification 
for this oinnion cannot, however, be furnished by the fact that in 
plants many characters have not been as yet explained by adapta- 
tion. We should do well to remember the extent to which the 
number of so-called ' morphological ' characters in plants has 
been lessened during the last twenty years. What a flood of 
light was thrown upon the forms and colours of flowers, so often 
ciu'ious and apparently arbitrary, when Sprengel's long-neglected 
discovery was extended and duly appreciated as the result of Dar- 
win's investigations, and when the subject was further advanced 
by Hermann Miiller's admirable researches ! Even the venation 
of leaves, which was formerly considered to be entirely without 
significance, has been shown to possess a high biological value 

' ' Ueber die Berechtigung der Darwin'schen Theorie.' Leipzig, iS68, p. 27. 
* 1. c, Preface, p. vi. 

S 2, 



260 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

by the ing'enious investig-ations of J. Sachs (see Appendix III, 
p. 308). We have not yet reached the limits of investig-ation, and no 
reason can be assigned for the belief that we shall not some day 
receive an explanation of characters which are now unintelligible^. 

It is obvious that the zoolog-ist cannot lay too much stress upon 
the intimate connexion between form and function, a connexion 
which extends to the minutest details : it is almost impossible to 
insist too much upon the perfect manner in which adaptation to 
certain conditions of life is carried out in the animal body. In 
the animal body we find nothing* without a meaning, nothing* 
which might be otherwise ; each organ, even each cell or part of 
a cell is, as it were, tuned for the special part it has to perform 
in relation to the surroundings. 

It is true that we are as yet unable to explain the adaptive 
character of every structure in any single species, but whenever 
we succeed in making out the significance of a structure, it 
always proves to be a fresh example of adaptation. Any one who 
has attempted to study the structure of a species in detail, and to 
account for the relation of its parts to the functions of the whole, 
will be altogether inclined to believe with me that everything 
depends upon adaptation. There is no part of the body of an 
individual or of any of its ancestors, not even the minutest and 
most insignificant part, which has arisen in any other way than 
under the influence of the conditions of life ; and the parts of the 
body conform to these conditions, as the channel of a river is 
shaped by the stream which flows over it. 

These are indeed only convictions, not real proofs ; for we are not 
yet sufficiently intimately acquainted with any species to be able 
to recognize the nature and meaning of all the details of its struc- 
ture, in all their relations : and we are still less able to trace the 
ancestral history in each case, and to make out the origin of those 
structures of which the presence in the descendants depends pri- 
marily upon heredity. But already a fair advance towards the 
attainment of inductive proof has been made ; for the number of 
adaptations which have been established is now very large and 

'■ Since the above was written many other morphological peculiarities of plants 
have been rightly explained as adaptations. Compare, for instance, the investiga- 
tions of Stahl on the means by which plants protect themselves against the attacks 
of snails and slugs (Jena, 1888),— A. W., 1888. 



IN THE THEORY OF NATURAL SELECTION. 261 

is increasing" every day. If, however, we anticipate the results of 
future researches, and admit that an org-anism only consists of 
adaptations, based upon an ancestral constitution, it is obvious 
that nothing remains to be explained by a phyletic force, even 
thoug-h the latter be presented to us in the refined form of Nageli's 
self-changing idioplasm. 

It will perhaps be useful to illustrate my views by a familiar 
example. I choose the well-known group of the whales. These 
animals are placental mammals, which, probably in secondary times, 
arose from terrestrial Mammalia, by adaptation to an aquatic life. 

Everything that is characteristic of these animals and distin- 
guishes them from other mammals depends upon this adaptation. 
Their fore-limbs have been transformed into rigid paddles, only 
movable at the shoulder-joint ; upon the back and the tail there 
are ridges with a form somewhat similar to the dorsal and caudal 
fins of fishes. The organ of hearing is without any external 
ear and without an air-containing external auditory meatus. The 
aerial vibrations do not pass, as in other mammals, from the ex- 
ternal auditory passage to the tympanic cavity and thus to the 
nerve-terminations of the inner ear ; but they reach the tympanic 
cavity by direct transmission through the bones of the skull, 
which possess a special structure and contain abundant air-cavities. 
This arrangement is obviously adapted for hearing in water. The 
nostrils also exhibit peculiarities, for they do not open near the 
mouth, but upon the forehead, so that the animal can breathe, 
even in a rough sea, as soon as it comes to the surface. In 
order to facilitate rapid movement in water, the whole body has 
become extended in length, and spindle-shaped, like the body of 
a fish. The hind limbs are absent in no other mammals, the fish- 
like Sirenia being alone excepted. In the whales, as in the Sirenia, 
these appendages have become useless, owing to the powerfully 
developed tail-fin ; they are now rudimentary and consist of some 
small bones and muscles deeply buried in the body of the animal, 
which nevertheless, in certain species, still exhibit the original 
structure of the hind-limb. The hairy covering of other mam- 
mals has also disappeared, its place having been taken by a thick 
layer of fat beneath the skin, which afibrds a much better pro- 
tection against cold. This fatty layer was also necessary in order 
to diminish the specific gravity of the animal, and to thus render 



262 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

it equal to that of sea-water. In the structure of the skull there 
are also a number of peculiarities, all of which are directly or in- 
directly connected with the conditions under which these animals 
live. In the whalebone whales, the enormous size of. the face, 
the immense jaws, and wide mouth are very striking. Can it 
be suggested that this very characteristic appearance is entirely 
due to the guidance of some internal transforming force, or to 
some spontaneous modification of the idioplasm ? Any such sug- 
gestion cannot be accepted, for it is easy to show that all these 
structural features depend upon adaptation to a peculiar mode 
of feeding. Functional teeth are absent, but rudimentary ones 
exist in the embryo as relics of an ancestral condition in which 
these organs were fully developed. Large plates of whalebone 
with finely divided ends are suspended vertically from the roof 
of the mouth. These whales feed upon small organisms, about an 
inch in length, which swim or float upon the water in countless 
numbers ; and in order that they may subsist upon such minute 
animals, it is necessary to obtain them in immense numbers. This 
is achieved by means of the huge mouth which takes in a vast 
quantity of water at a single mouthful. The water then filters 
away through the plates of whalebone, while the organisms which 
form the whale's food remain stranded in the mouth. Is it neces- 
sary to add that the internal organs — so far as we understand the 
details of their functions, and so far as their structure differs from 
that of the corresponding organs in other Mammalia — have also 
been directly or indirectly modified by adaptation to an aquatic 
life ? Thus all whales possess a very peculiar arrangement of 
the nasal passages and larynx, enabling them to breathe and 
swallow at the same time : the lungs are of enormous length, and 
thus cause the animal to assume a horizontal position in the water 
without the exercise of muscular effort: in consequence of this 
latter modification, the diaphragm extends in a nearly horizontal 
direction : there are moreover certain an-angements in the vascular 
system which enable the animal to remain under water for a con- 
siderable time, and so on. 

And now, in reference to this special example, I will repeat 
the question which I have asked before : — ' If everything that 
is characteristic of a group of animals depends upon adaptation, 
what remains to be explained by the operation of an internal 



IN THE THEORY OF NATURAL SELECTION. 263 

developmental force ? ' What remains of a whale when we have 
taken away its adaptive characters ? We are compelled to reply 
that nothing- remains except the general plan of mammalian 
organization, which existed previously in the mammalian ancestors 
of the Cetacea. But if everything which stamps these animals as 
whales has arisen by adaptation, it follows that the internal de- 
velopmental force cannot have had any share in the origin of this 
group. 

And yet this very force is said to be the main factor in the 
transformation of species, and Nageli unhesitatingly asserts that 
both the animal and vegetable kingdoms would have become very 
much as they now are, if there had been no adaptation to new 
conditions, and no such thing as competition in the struggle for 
existence ^. 

But even if we admit that such an assumption affords some 
explanation, instead of being the renunciation of all attempts at 
explanation ; if we admit that an organism, the characteristic 
peculiarities of which entirely depend upon adaptation, has been 
formed by an internal developmental force ; we should still be 
unable to explain how it happens that such an organism, suited to 
certain conditions of life, and unable to exist under other conditions, 
appeared at that very place on the earth's surface, and at that very 
time in the earth's history, which offered the conditions appropriate 
for its existence. As I have previously argued, the believers in 
an internal developmental force are compelled to invent an auxiliary 
hypothesis, a kind of 'pre-established harmony' which explains 
how it is that changes in the organic world advance step by step, 
parallel with changes in the crust of the earth and in other 
conditions of life ; just as, according to Leibnitz, body and soul, 
although independent of each other, proceed along* parallel courses, 
like two chronometers which keep perfect time. And even this 
supposition would not be sufficient, because the place must be 
taken into account as well as the time : thus the whales could not 
have existed if they had first appeared upon dry land. We know 
of countless instances in which a species is exclusively and precisely 
adapted to a certain localized area, and could not thrive anywhere 
else. We have only to remember the cases of mimicry in which 
one insect gains protection by resembling another, the cases of 

^ 1. c, pp. 117, 286. 



264 THE SIGNIFICANCE OP SEXUAL EEPRODUCTION 

protective resemblance to the bark or the leaves of a certain species 
of plant, or the numerous marvellous adaptations of parasitic 
animals to certain parts of certain species of hosts. 

A mimetic species cannot have appeared at any place other 
than that in which it exists : it cannot have arisen through an 
internal developmental force. But if single species, or even whole 
orders like the Cetacea, have arisen independently of any such force, 
then we may safely assert that the existence of the supposed force 
is neither required by reason nor necessity. 

Hence, abstaining from the invocation of unknown forces, we are 
justified in carrying on Darwin's attempt to explain the trans- 
formation of organisms by the action of known forces and known 
phenomena. I say ' carry on the attempt,' because I do not believe 
that our knowledge in this direction has ended with Darwin, and 
it seems to me that we have already arrived at ideas which are in- 
compatible with certain important points in his general theory, and 
which therefore necessitate some modification of the latter. 

The theory of natural selection explains the rise of new species 
by supposing that changes occur, from time to time, in those con- 
ditions of life to which an organism must adapt itself if it is to 
continue in existence. Thus a selective process is set up which 
ensures that only those out of the existing variations are pre- 
served, which correspond in the highest degree to the changed 
conditions of life. By continued selection in the same direction 
the deviations fiom the type, although at first very insignificant, 
are accumulated and increased until they become specific differ- 
ences. 

I should wish to assert more definitely than Darwin has done, 
that alterations in the conditions of life, together with changes in 
the organism itself, must have advanced very gradually and by the 
smallest steps, in such a way that, at each period in the whole pro- 
cess of transformation, the species has remained sufficiently adapted 
to the surrounding conditions. An abrupt transformation of a species 
is inconceivable, because it would render the species incapable of 
existence. If the whole organization of an animal depends upon 
adaptation, if the animal body is, as it were, an extremely complex 
combination of new and old adaptations, it would be a highly 
remarkable coincidence if, after any sudden alteration occurring 
simultaneously in many parts of the body, all these parts were 



IN THE THEOEY OP NATUEAL SELECTION. 265 

changed in such a manner that they again formed a whole which 
exactly corresponded to the altered external conditions. Those who 
assume the existence of such a sudden transformation overlook the 
fact that everything in the animal body is exactly calculated to 
maintain the existence of the species, and that it is just sufficient 
for this purpose ; and they forget that the minutest change in the 
least important organ may be enough to render the species in- 
capable of existence. 

It may perhaps be objected that the case is different in plants, as 
is proved by the American weeds which have spread all over 
Europe, or the European plants which have become naturalized in. 
Australia. Reference might also be made to the plants which 
inhabited the plains during the glacial epoch, and which at its 
close migrated to the Alpine mountains and to the far north, and 
which have remained unaltered under the apparently diverse con- 
ditions of life to which they have been subjected for so long a 
time. Similar instances may also be found among animals. The 
rabbit, which was brought by sailors to the Atlantic island of Porto 
Santo, has bred abundantly and remains unchanged in this locality ; 
the European frogs, which were introduced into Madeira, have in- 
creased immensely and have become almost a plague ; and the 
European sparrow now thrives in Australia quite as well as with us. 
But these instances do not prove that adaptation to external 
conditions of life is not of primary importance ; they do not prove 
that an organism which is adapted to a certain environment will, 
when unmodified, remain capable of existence amid new surround- 
ings. They only prove that the above-mentioned species found 
in those countries the same conditions of life as at home, or at 
least that they met with conditions to which their organization 
could be subjected without the necessity for modification. Not 
every new environment includes such changed conditions as will be 
effective in modifying every species of plant or animal. The rabbit 
of Porto Santo certainly feeds on herbs different from those which 
form the food of its relations in Europe, but such a change does 
not mean an effective alteration in the conditions under which this 
species lives, for the herbs in both localities are equally well suited 
to the needs of the animal. 

But if we suppose that the wild rabbit, occurring in Europe, were 
to suddenly lose but a trifle of its wariness, its keen sight, its fine 



•^ 



266 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

sense of hearing or of smell, or were to suddenly acquire a colour 
different from that which it now possesses, it would become in- 
capable of existence as a species, and would soon die out. The same 
result would probably occur if any of its internal org-ans, such as 
the lungs or the liver, were suddenly modified. Perhaps single 
individuals would still remain capable of existence under these cir- 
cumstances, but the whole species would suffer a certain decline 
from the maximum development of its powers of resistance, and 
would thus become extinct. The sudden transformation of a species 
appears to me to be inconceivable from a physiological point of 
view, at any rate in animals. 

Hence the transformation of a species can only take place by the 
smallest steps, and must depend upon the accumulation of those 
differences which characterise individuals, or, as we call them, 
' individual differences.' There is no doubt that these differences 
are always present, and thus, at first sight, it appears to be simply 
a matter of course that they will afford the material by means of 
which natural selection produces new forms of life. But the case is 
not so simple as it appeared to be until recently; that is if I am 
right in believing that in all animals and plants which are repro- 
duced by true germs, only those characters which were potentially 
present in the germ of the parent can be transmitted to the 
succeeding generation. 

I believe that heredity depends upon the fact that a small portion 
f the effective substance of the germ, the germ-plasm, remains 
unchanged during the development of the ovum into an organism, 
and that this part of the germ-plasm serves as a foundation from 
which the germ-cells of the new organism are produced ^ There is 
therefore continuity of the germ-plasm from one generation to 
another. One might represent the germ-plasm by the metaphor of 
a long creeping root-stock from which plants arise at intervals, 
these latter representing the individuals of successive generations. 

Hence it follows that the transmission of acquired characters is an 
impossibility, for if the germ-plasm is not formed anew in each 
individual but is derived from that which preceded it, its structure, 
and above all its molecular constitution, cannot depend upon the 
individual in which it happens to occur, but such an individual 

' Compare the second and fourth of the preceding Essays, ' On Heredity ' and ' The 
Continuity of the Germ-plasm as the Foundation of a Theory of Heredity.' 



IN THE THEORY OF NATURAL SELECTION. 267 

only forms, as it were, the nutritive soil at the expense of which the 
germ-plasm grows, while the latter possessed its characteristic struc- 
ture from the beginning, viz. before the commencement of growth. 

But the tendencies of heredity, of which the germ-plasm is the 
bearer, depend upon this very molecular structure, and hence only 
those characters can be transmitted through successive generations 
which have been previously inherited, viz. those characters which 
were potentially contained in the structure of the germ-plasm. It 
also follows that those other characters which have been acquired by 
the influence of special external conditions, during the life-time of 
the parent, cannot be transmitted at all. 

The opposite view has, up to the present time, been maintained, 
and it has been assumed, as a matter of course, that acquired 
characters can be transmitted ; furthermore, extremely complicated 
and artificial theories have been constructed in order to explain how 
it may be possible for changes produced by the action of external 
influences, in the course of a life-time, to be communicated to the 
germ and thus to become hereditary. But no single fact is known 
which really proves that acquired characters can be transmitted, 
for the ascertained facts which seem to point to the transmission of 
artificially produced diseases cannot be considered as a proof; and 
as long as such proof is wanting we have no right to make this 
supposition, unless compelled to do so by the impossibility of 
suggesting a mode in which the transformation of species can take 
place without its aid. (See Appendix IV, p. 310.) 

It is obvious that the unconscious conviction that we need the 
aid of acquired characters has hitherto securely maintained the as- 
sumed axiom of the transmission of such features. It was believed 
that we could not do without such an axiom in order to explain the 
transformation of species ; and this was believed not only by those 
who hold that the direct action of external influences plays an 
important part in the process, but also by those who hold that the 
operation of natural selection is the main factor. 

Individual variability forms the most important foundation of 
the theory of natural selection : without it the latter could not 
exist, for this alone can furnish the minute differences by the; 
accumulation of which new forms are said to arise in the course of 
generations. But how can such hereditary individual characters 
exist if the changes wrought by the action of external influences, 



268 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

during the life of an individual, cannot be transmitted ? We are 
clearly compelled to find some other source of hereditary in- 
dividual differences, or the theory of natural selection would collapse, 
as it certainly would if hereditary individual variations did not 
exist. If, on the other hand, acquired differences are transmitted, 
this would prove that there must be something wrong in the 
theory of the continuity of the germ-plasm, as above described, and 
in the non- transmission of acquired characters which results from 
this theory. But I believe that it is possible to suggest that the 
origin of hereditary individual characters takes place in a manner 
quite different from any which has been as yet brought forward. 
To explain this origin is the task which I am about to undertake 
in the following pages. 

The origin of individual variability has been hitherto represented 
somewhat as follows. The phenomena of heredity lead to the 
conclusion that each organism is capable of producing germs, from 
which, theoretically at least, exact copies of the parent may arise. 
In reality this is never the case, because each organism possesses 
the power of reacting on the different external influences with 
which it is brought into contact, a power without which it 
could neither develope nor exist. Each organism reacting in a 
different way must be to some extent changed. Favoiu^ble nutri- 
tion makes such an organism strong and large ; unfavourable nu- 
trition renders it small and weak, and what is true of the whole 
organism may also be said of its parts. Now it is obvious that 
even the children of the same mother meet with influences different 
in kind and degree, from the very beginning of their existence, so 
that they must necessarily become unlike, even if we suppose them 
to have been derived from absolutely identical germs, with precisely 
the same hereditary tendencies. 

In this manner individual differences are believed to have been 
introduced. But if acquired characters are not transmitted the 
whole chain of argument collapses, for none of those changes which 
are caused by the- conditions of nutrition acting upon single parts 
of the whole organism, including the results of training and of the 
use or disuse of single organs, — none of these changes can furnish 
hereditary differences, nor can they be transmitted to succeeding 
generations. They are, as it were, only transient characters as far 
as the species is concerned. 



IN THE THEOKY OF NATURAL SELECTION. 269 

The children of accomplished pianists do not inherit the art of 
playing" the piano ; they have to learn it in the same laborious 
manner as that by which their parents acquired it ; they do not 
inherit anything* except that which their parents also possessed 
when children, viz. manual dexterity and a good ear. Fui-thermore, 
language is not transmitted to our children, although it has been 
practised not only by ourselves but by an almost endless line of 
ancestors. Only recently, facts have again been worked up and 
brought together, which show that children of highly civilized 
nations have no trace of a language wken they have grown up in 
a wild condition and in complete isolanon ^. The power of speech 
is an acquired or transient character : it is not inherited, and cannot 
be transmitted : it disappears with the organism which manifests it. 
Not only do similar phenomena occur in the vegetable kingdom, 
but they present themselves in an especially striking manner. 

When Nageli ^ introduced Alpine plants, taken from their natural 
habitat, into the botanical g-arden at Munich, many of the species 
were so greatly altered that they could hardly be recog-nized : for 
instance, the small Alpine hawk-weeds became large and thickly 
branching, and they blossomed freely. But if such plants, or even 
their descendants, were removed to a poor g-ravelly soil the new 
characters entirely disappeared, and the plants were re-transformed 
into the original Alpine form. The re-transformation was always 
complete, even when the species had been cultivated in rich garden 
soil for several generations. 

Similar experiments with identical results were made twenty 
years ago by Alexis Jordan ^, who chiefly made use of Draba verna 
in his researches. These experiments furnish very strong proofs, 
because they were originally undertaken without the bias which 
may be ^iven by a theory. Jordan only intended to decide experi- 
mentally whether the numerous forms of the plant, as it occurs 
wild in different habitats, are mere varieties or true species. He 
found that the different forms do not. pass into one another, and 

^ Compare Rauber, ' Homo sapiens ferus oder die Zustlinde der Verwilderten.' 
Leipzig, 1885. 

^ ' Sitzungsberichte der baierischen Akademie der Wissenschaften,' vom 18 Nov. • 
1865. Compare also his ' Mechanisch-physiologische Theorie der Abstammungslehre/ 
p. 102, etc. 

^ Jordan, ' Eemarques sur le fait de I'existence en societe des espfeces vegetales 
affines.' Lyon, 1873. 



270 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

are in all cases re-transformed after they have been altered by culti- 
vation in a soil different from that'^ii which they usually g-row, 
and he therefore assumed that they were true species. All these 
experiments therefore confirm the conclusion that external influences 
may alter the individual, but that the changes produced are not 
transmitted to the germs, and are never hereditary. 

Naareli indeed asserts that innate individual differences do not 
exist in plants. The differences which we find, for instance, be- 
tween two beeches or oaks, are always, according to him, modifi- 
cations produced by the influence of varying local conditions. But 
it is obvious that Nageli goes too far in this respect, although it 
may be conceded that innate individual differences in plants are 
much more difficult to distinguish from those which are acquired, 
than in animals. 

There is no doubt about the occurrence of innate and hereditary 
individual characters in animals, and we may find an especially 
interesting illustration in the case of man. The human eye can 
with practice appreciate the most minute differences between indi- 
vidual men, and especially differences of featui'e. Every one knows 
that peculiarities of featm-e persist in certain families through a 
long series of generations. I need hardly remind the reader of the 
broad forehead of the Julii, the projecting chin of the Hapsburgs, 
or the curved nose of the Bourbons. Hence every one can see that 
hereditary individual characters do unquestionably exist in man. The 
same conclusion may be affirmed with equal certainty for all our 
domestic animals, and I do not see any reason why there should 
be any doubt about its application to other animals and to plants. 

But now the question arises, — How can we explain the presence 
of such characters consistently with a belief in the continuity of the 
germ-plasm, a theory which implies the rejection of the supposition 
that acquired characters can become hereditary? How can the 
individuals of any species come to possess various characters 
which are undoubtedly hereditary, if all changes which are due to 
the influence of external conditions are transient and disappear 
with the individual in which they arose ? Why is it that in- 
dividuals are distinguished by innate characters, as well as by those 
which I have previously called transient, and how can deep-seated 
hereditary characters arise at all, if they are not produced by the 
external influences to which the individual is exposed ? 



IN THE THEORY OF NATURAL SELECTION. 271 

In the first place it may be arg-ued that external influences may 
not only act on the mature individual, or during* its development, 
but that they may also act at a still earlier period upon the g'erm- 
cell from which it arises. It may be imag-ined that such influences 
of difFerent kinds might produce corresponding- minute alterations 
in the molecular structure of the g-erm-plasm, and as the latter is, 
according" to our supposition, transmitted from one g-eneration to 
another, it follows that such chang-es would be hereditary. 

Without altogether denying* that such influences may directly 
modify the germ-cells, I nevertheless believe that they have no 
share in the production of hereditary mdividual characters. 

The germ-plasm or idioplasm of the germ-cell (if this latter term 
be preferred) certainly possesses an exceedingly complex minute 
structure, but it is nevertheless a substance of extreme stability, for it 
absorbs nourishment and grows enormously without the least change 
in its complex molecular structure. With Niigeli we may indeed 
safely affirm so much, although we are unable to acquire any direct 
knowledge as to the constitution of germ-plasm. When we know 
that many species have persisted unchanged for thousands of years, 
we have before us the proof that their germ-plasm has preserved 
exactly the same molecular structure during the whole period. I 
may remind the reader that many of the embalmed bodies of the 
sacred Egyptian animals must be fom- thousand years old, and that 
the species are identical with those now existing in the same 
locality. Now, since the quantity of germ-plasm contained in a 
single germ-cell must be very minute, and since only a very small 
fraction can remain unchanged when the germ-cell developes into 
an organism, it follows that an enormous growth of this small 
fraction must take place in every individual, for it must be re- 
membered that each individual produces thousands of germ-cells. 
It is therefore not too much to say that, during a period of four 
thousand years, the growth of the g^rm-plasm in the Egyptian ibis 
or crocodile must have been quite stupendous. But in the animals 
and plants which inhabit the Alps and the far north, we have 
instances of species which have remained unchanged for a much 
longer period, viz. for the time which has elapsed between the close 
of the glacial epoch and the present day. In such organisms the 
growth of the germ-plasm must therefore have been still greater. 

If nevertheless the molecular structure of the germ-plasm has 



272 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

remained precisely the same, this substance cannot be readily 
modifiable, and there is very little chance of the smallest changes 
being produced in its molecular structure, by the operation of those 
minute transient variations in nutrition to which the germ-cells, 
together wdth every other part of the organism, are exposed. The 
rate of growth of the germ-plasm will certainly vary, but its struc- 
ture is unlikely to be affected for the above-mentioned reasons, and 
also because the influences are mostly changeable, and occur some- 
times in one and sometimes in another direction. 

Hereditary individual differences must therefore be derived from 
some other source. 

I believe that such a source is to be looked for in the form 
of reproduction by which the great majority of existing organisms 
are propagated : viz. in sexual, or, as Hackel calls it, amphigonic 
reproduction. 

It is well known that this process consists in the coalescence of 
two distinct germ-cells, or perhaps onjy of .their nuclei. These 
germ-cells contain the germ-substance, the germ-plasm, and this 
again, owing to its specific molecular structure, is the bearer of the 
hereditary tendencies of the organism from which the germ-cell 
has been derived. Thus in amphigonic reproduction two groups 
of hereditary tendencies are as it were combined. I regard this 
combination as the cause of hereditary individual characters, and 
I believe that the production of such characters is the true sig- 
nificance of amphigonic reproduction. The object of this process is 
to create those individual differences which form the material out of 
which natural selection produces new species. 

At first sight this conclusion appears to be very startling and 
almost incredible, because we are on the contrary inclined to believe 
that the continued combination of existing differences, which is 
implied by the very existence of amphigonic reproduction, cannot 
lead to their intensification, but rather to their diminution and 
gradual obliteration. Indeed the opinion has already been ex- 
pressed that deviations from the specific type are rapidly destroyed 
by the operation of sexual reproduction. Such an opinion may be 
true with regard to specific characters, because the deviations from 
a specific type occur in such rare cases that they cannot hold their 
ground against the large number of normal individuals. But the 
case is different with those minute differences which are characteristic 



IN THE THEOKY OP NATUKAL SELECTION. 273 

of individuals, because every individual possesses them, althoug-h 
of a different kind and degree. The extinction of such dif- 
ferences could only take place if a few individuals constituted a 
whole species ; but the number of individuals which together 
represent a species is not only very large but generally incalculable. 
Cross-breeding between all individuals is impossible, and hence the 
obliteration of individual differences is also impossible. 

In order to explain the effects of sexual reproduction, we will 
first of all consider what happens in monogenic or unisexual re- 
production, which actually occurs in parthenogenetic organisms. 
Let us imagine an individual producing germ-cells, each of which 
may by itself develope into a new individual. If we then suppose 
a species to be made up of individuals which are absolutely identical, 
it follows that their descendants must also remain identical through 
any number of generations, if we neglect the transient non- 
transmissible peculiarities caused by differences of food and other 
external conditions. 

Although the individuals of such a species might be actually 
different, they would be potentially identical : in the mature state 
they might differ, but they must have been identical in origin. 
The germs of all of them must contain exactly the same hereditary 
tendencies, and if it were possible for their development to take 
place under exactly the same conditions, identical individuals would 
be produced. 

Let us now assume that the individuals of such a species, repro- 
ducing itself by the monogonic process and therefore without cross- 
breeding, differ, not only in transient but also in hereditary cha- 
racters. If this were the case, each individual would produce 
descendants possessing the same hereditary differences which were 
characteristic of itself ; and thus from each individual a series of 
generations would emanate, the single individuals of which 
would be potentially identical with each other and with their 
first ancestor. Hence the same individual differences would be 
repeated again and again, in each succeeding generation, and 
even if all the descendants lived to reproduce themselves, there 
would be at last just as many groups of potentially identical 
individuals as there were single individuals at the beginning. 

Similar cases actually occur in many species in which sexual 
reproduction has been entirely replaced by the parthenogenetic 



274 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

method, as in many species of Cynip% and in certain lower Crustacea. 
But all these difler from our hypothetical case in one important 
respect ; it is always impossible for all the descendants to reach 
maturity and reproduce themselves. The vast majority of the 
descendants g-enerally perish at an early stage, and only about as 
many remain to continue the species as reached maturity in the 
preceding" generation. 

We have now to consider whether such a species can be subject 
to the operation of natural selection. Let us take the case of an 
insect living among green leaves, and possessing a green colour as 
a protection against discovery by its enemies. We will assume that 
the hereditary individual differences consist of various shades of 
green. Let us further suppose that the sudden extinction of its 
food-plant compelled this species to seek another plant with a 
somewhat different shade of green. It is clear that such an insect 
would not be completely adapted to the new environment. It 
would therefore be compelled, metaphorically speaking, to en- 
deavour to bring its colour into closer harmony with that of the 
new food-plant, or else the increased chances of detection given to 
its enemies would lead to its slow but cei^tain extinction. 

It is obvious that such a species would be altogether unable to 
produce the required adaptation, for ex ht/pothesi^ its hereditary 
variations remain the same, one generation after another. If 
therefore the required shade of green was not previously present, 
as one of the original individual differences, it could not be pro- 
duced at any time. If, however, we suppose that such a colour 
existed previously in certain individuals, it follows that those with 
other shades of green would be gradually exterminated, while 
the former would aloUe survive. But this process would not be 
an adaptation in the sense used in the theory of natural selection. 
It would indeed be a process of selection, but it could form no 
more than the beginning of that process which we call natural 
selection. If the latter could only bring existing characters into 
prominence, it would not be worth much consideration, for it could 
never produce a new species. A species never includes, from the 
beginning, individuals which deviate from the specific type as 
widely as the individuals of the most nearly allied species 
deviate from it. And it would be still less possible to explain, 
on such a principle, the origin of the whole organic world ; for, if 



IN THE THEORY OF NATURAL SELECTION. 275 

SO, all existing" species would have been included as variations of 
the first species. Natural selection must be able to do infinitely 
more than this, if it is to be of any importance as a principle of 
development. It must be able to accumulate minute existing- dif- 
ferences in the required direction, and thus to create new characters. 
In our example it ought to be able, after preserving those in- 
dividuals with a colour nearest to the required shade, to lead their 
descendants onward through successive stages towards a complete 
harmony of colour. 

But such a result is quite unattainable with the asexual method 
of reproduction : in other words, natural selection, in the true 
meaning of the term, viz. a process which could produce new 
characters in the manner above described, is an impossibility in a 
species propagated by asexual reproduction. 

If it could be shown that a purely parthenogenetic species had 
become transformed into a new one, such an observation would 
prove the existence of some force of transformation other than 
selective processes, for the new species could not have been pro- 
duced by these latter. As already explained, the only selection 
which would be possible for such a species, would lead to the 
sm-vival of one group of individuals and to the extinction of all 
others. Thus in our example that group of individuals would 
alone survive, the ancestors of which originally possessed the 
appropriate colour. But if one group alone survived, it follows 
that all hereditary individual differences would have disappeared 
from the species, for the members of such a single group are 
identical with one another and with their original ancestors. We 
thus reach the conclusion that monogenic reproduction can never 
cause hereditary individual variability, but that, on the other hand, 
it is very likely to lead to its entire suppression. 

But the case is very different with sexual reproduction. When 
once individual differences have begun to appear in a species pro- 
pagated by this process, uniformity among its individuals can 
never again be reached. So far from this being the case, the 
differences must even be increased in the com-se of generations, not 
indeed in intensity, but in number, for new combinations of the 
individual characters will continually arise. 

Again, assuming the existence of a number of individuals which 
differ from one another by a few hereditary individual characters, 

T 3 



276 THE SIGNIFICANCE OF SEXUAL REPKODUCTION 

it follows that no individual of the second generation can be iden- 
tical with any other. They must all differ, not only actually but 
also potentially, for their differences exist at the very beginning' of 
develoi^ment, and do not solely depend upon the accidental conditions 
under which they live. Moreover, no one of the descendants can 
be identical with any of the ancestors, for each of the former unites 
within itself the hereditary tendencies of two parents, and its 
organism is therefore, as it were, a compromise between two de- 
velopmental tendencies. Similarly in the third generation, the 
hereditary tendencies of two individuals of the second generation 
enter into combination. But since the germ-plasm of the latter 
is not simple, but composed of two individually distinct kinds of 
germ-plasm, it follows that an individual of the third generation 
is a compromise between four different hereditary tendencies. In 
the fourth generation, eight ; in the fifth, sixteen ; in the sixth, 
thirty-two different hereditary tendencies must come together, and 
each of them will make itself more or less felt in some part of the 
future organism. Thus by the sixth generation a large number of 
varied combinations of ancestral individual characters will appear, 
combinations which have never existed before and which can never 
exist again. 

We do not know the number of generations over which the 
specific hereditary tendencies of the first generation can make 
themselves felt. Many facts seem to indicate however that the 
number is large, and it is at all events greater than six. When 
we remember that, in the tenth generation, a single germ contains 
1024 different germ-plasms, with their inherent hereditary ten- 
dencies, it is quite clear that continued sexual reproduction can 
never lead to the re-appearance of exactly the same combination, 
but that new ones must always arise. 

New combinations are all the more probable because the different 
idioplasms composing the germ-plasm in the germ-cells of any 
individual are present in different degrees of intensity at different 
times of its life ; in other words, the intensity of the component 
idioplasms is a function of time. This conclusion follows from the 
fact that children of the same parents are never exactly identical. 
In one child the characters of the father may predominate, in 
another those of the mother, in another again those of either 
grand-parent or great-grand-parent. 



IN THE THEORY OF NATURAL SELECTION. 277 

We are thus led to the conchision that even in a few sexually 
produced generations a large number of well-marked individuals 
must arise : and this would even he true of generations springing 
from our hypothetical species, assumed to be without ancestors, and 
characterised by few individual differences. But of course organ- 
isms which reproduce themselves sexually are never without 
ancestors, and if these latter were also propagated by the sexual 
method, it follows that each generation of every sexual species is 
in the stage which we h^ve previously assumed for the tenth 
or some much later generation of the hypothetical species. In 
other words, each individual contains a maximum of hereditary 
tendencies and an infinite variety of possible individual characters 
(see Appendix VI, p. 326). 

In this manner we can explain the origin of hereditary in- 
dividual variability as it is known in man and the higher animals, 
and as it is required for the theory which explains the transformation 
of species by means of natural selection. 

Before proceeding fui'ther, I must attempt to answer a question 
which obviously suggests itself. For the sake of argument, I 
have assumed the existence of a first generation, of which the 
individuals were already characterised by individual differences. 
Can we find any explanation of these latter, or are we compelled 
to take them for granted, without any attempt to enquire into their 
origin ? If we abandon this enquiry, we can never achieve a com- 
plete solution of the problems of heredity and variability. We 
have, it is true, shown that hereditary differences, when they have 
once appeared, would, through sexual reproduction, undergo de- 
velopment into the diverse forms which actually exist ; but this 
conclusion affords us no explanation of the source whence such 
differences have been derived. If the external conditions acting 
directly upon an organism can only produce transient (viz. non- 
hereditary) differences in the latter, and if, on the other hand, the 
external influences which act upon the germ-cell can only produce 
a change in its molecular structure after operating over very long 
periods, it seems that we have exhausted all the possible sources 
of hereditary differences without reaching any satisfactory ex- 
planation. 

I believe, however, that an explanation can be given. The origin 
of hereditary individual variability cannot indeed be found in the 



278 THE SIQNIFICANOE OF SEXUAL REPRODUCTION 

higher organisms — the Metazoa and Metaphyta; but it is to be 
sought for in the lowest — the unicellular organisms. In these 
latter the distinction between body-cell and germ-cell does not exist. 
Such organisms are reproduced by division, and if therefore any 
one of them becomes changed in the course of its life by some 
external influence, and thus receives an individual character, the 
method of reproduction ensures that the acquired peculiarity will 
be transmitted to its descendants. If, for instance, a Protozoon, by 
constantly struggling against the mechanical influence of currents 
in water, were to gain a somewhat denser and more resistent 
protoplasm, or were to acquire the power of adhering more strongly 
than the other individuals of its species, the peculiarity in question 
would be directly continued on into its two descendants, for the 
latter are at first nothing more than the two halves of the former. 
It therefore follows that every modification which appears in the 
course of its life, every individual character, however it may have 
arisen, must necessarily be directly transmitted to the two ofi"- 
spring of a unicellular organism. 

The pianist, whom I have already used as an illustration, may 
by practice develope the muscles of his fingers so as to ensure the 
highest dexterity and power ; but such an effect would be entirely 
transient, for it depends upon a modification in local nutrition 
which would be unable to cause any change in the molecular 
structure of the germ-cells, and could not therefore produce any 
effect upon the offspring. And even if we admit that some change 
might be caused in the germ-cells, the chances would be infinity 
to nothing against the production of the appropriate effect, viz. 
such a change as would lead to the development in the child of 
the acquired characters of the parent. 

In the lowest unicellular organisms, however, the case is en- 
tirely different. Here parent and offspring are still, in a certain 
sense, one and the same thing : the child is a part, and usually 
half, of the parent. If therefore the individuals of a unicellular 
species are acted upon by any of the various external influences, 
it is inevitable that hereditary individual differences will arise in 
them ; and as a matter of fact it is indisputable that changes are 
thus produced in these organisms, and that the resulting characters 
are transmitted. It has been directly observed that individual 
differences do occur in unicellular organisms, — differences in size, 



IN THE THEORY OF NATURAL SELECTION. 279 

colour, form, and the number or arrangement of cilia. It must be 
admitted that we have not hitherto paid sufficient attention to 
this point, and moreover our best microscopes are only very roug-h 
means of observation when we come to deal with such minute 
organisms. Nevertheless we cannot doubt that the individuals 
of the same species are not absolutely identical. 

We are thus driven to the conclusion that the ultimate origin of 
hereditary individual differences lies in the direct action of ex- 
ternal influences upon the organism. Hereditary variability can- 
not however arise in this way at every stage of organic develop- 
ment, as biologists have hitherto been inclined to believe. It can 
only arise in the lowest unicellular organisms ; and when once 
individual difference had been attained by these, it necessarily 
passed over into the higher organisms when they first appeared. 
Sexual reproduction coming into existence at the same time, the 
hereditary differences were increased and multiplied, and arranged 
in ever-changing combinations. 

Sexual reproduction can also increase the differences between 
individuals, because constant cross-breeding must necessarily and 
repeatedly lead to a combination of forces which tend in the 
same direction, and which may determine the constitution of any 
part of the body. If, for instance, the same part of the body is 
strongly developed in both parents, the experience of breeders tells 
us that the part in question is likely to be even more strongly 
developed in the offspring ; and that weakly developed parts will 
in the same manner tend to become still weaker. Amphigonic 
reproduction therefore ensures that every character which is sub- 
ject to individual fluctuation must appear in many individuals 
with a strengthened degree of development, in many others with 
a development which is less than normal, while in a still larger 
number of individuals the average development will be reached. 
Such differences afford the material by means of which natural 
selection is able to increase or weaken each character according 
to the needs of the species. By the removal of the less well- 
adapted individuals, natural selection increases the chance of 
beneficial cross-breeding in the subsequent generations. 

Every one must admit that, if a species came into existence 
having only a small number of individual differences which 
appeared in the different parts of different individuals, the number 



280 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

of differences would increase with each sexually produced generation, 
until all the parts in which the variations occurred had received 
a peculiar character in all individuals. 

Moreover sexual reproduction not only adds to the number of 
existing" differences, but it also brings them into new combina- 
tions, and this latter consequence is as important as the former. 

The former consequence can hardly make itself felt in any 
existing- species, because in them every part already possesses its 
peculiar character in all individuals. The second consequence is, 
however, more important, viz. the production of ijew combina;- 
tions of individual characters by sexual reproduction; for, as 
Darwin has already pointed out, we must imagine that not only 
are single characters changed in the process of breeding, but that 
probably several, and perhaps very many characters, are simul- 
taneously modified. No two species, however nearly allied, differ 
from each other in but a single character. Even our eyesight, 
which has by no means reached the highest pitch of development, 
can always detect several, and often very many points of difference ; 
and if we possessed the powers necessary for making an absolutely 
accurate comparison, we should probably find that everything is 
different in two nearly allied species. 

It is true that a great number of these differences depend upon 
correlation, but others must depend upon simultaneous primary 
changes. 

A large butterfly {Kallima paraleda), found in the East Indian 
forests, has often been described in its position of rest as almost 
exactly resembling a withered leaf ; the resemblance in colour 
being aided by the markings which imitate the venation of a leaf. 
These markings are composed of two parts, the upper of which is 
on the fore-wings, while the lower one is on the hind wings. The 
butterfly when at rest must therefore keep the wings in such a 
position that the two parts of each marking exactly correspond, 
for otherwise the character would be valueless ; and as a matter of 
fact the wings are held in the appropriate position, although the 
butterfly is of course unconscious of what it is doing. Hence a 
mechanism must exist in the insect's brain which compels it to 
assume this attitude, and it is clear that the mechanism cannot have 
been developed before the peculiar manner of holding the wings 
became advantageous to the butterfly, viz. before the similarity 



IN THE THEORY OF NATURAL SELECTION. 281 

to a leaf had made its first appearance. Conversely, this latter 
resemblance could not develope before the butterfly had gained the 
habit of holding" its wings in the appropriate position. Both 
characters must therefore have come into existence simultaneously, 
and must have undergone increase side by side : the marking 
progressing from an imperfect to a very close similarity, while 
the position of the wings gradually approached the attitude 
which was exactly appropriate. The development of certain 
minute structural elements of the central nervous system, and the 
appropriate distribution of colouring matter on the wings, must 
have taken place simultaneously, and only those individuals have 
been selected to continue the species which possessed the favourable 
variations in both these directions. 

It is, however, obvious that sexual reproduction will readily 
afford such combinations of required characters, for by its means 
the most diverse features are continually united in the same indi- 
vidual, and this seems to me to be one of its most important 
results. 

I do not know what meaning can be attributed to sexual repro- 
duction other than the creation of hereditary individual characters 
to form the material upon which natural selection may work. 
Sexual reproduction is so universal in all classes of multicellular 
organisms, and nature deviates so rarely from it, that it must 
necessarily be of pre-eminent importance. If it be true that 
new species are produced by processes of selection, it follows that 
the development of the whole organic world depends on these pro- 
cesses, and the part that amphigony has to play in nature, by 
rendering selection possible among multicellular organisms, is not 
only important, but of the very highest imaginable importance. 

But when I maintain that the meaning of sexual reproduction is 
to render possible the transformation of the higher organisms by 
means of natural selection, such a statement is not equivalent to 
the assertion that sexual reproduction originally came into exist- 
ence in order to achieve this end. The effects which are now pro- 
duced by sexual reproduction did not constitute the causes which 
led to its first appearance. Sexual reproduction came into existence 
before it could lead to hereditary individual variability. Its first 
appearance must therefore have had some other cause ; but the 
nature of this cause can hardly be determined with any degree of 



282 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

certainty or precision from the facts with which we are at present 
acquainted. The g-eneral solution of the problem will, however, 
be found to lie in the conjugation of unicellular organisms, which 
forms the precursor of true sexual reproduction. The coalescence 
of two unicellular individuals which represents the simplest and 
therefore probably the most primitive form of conjugation, must 
have some directly beneficial effect upon the species in which it 
occurs. 

Various assumptions may be made as to the nature of these bene- 
ficial effects, and it will be useful to consider in detail some of 
those suggestions which have been brought forward. Eminent 
biologists, such as Victor Hensen ^ and Edouard van Beneden -, 
believe that conjugation, and indeed sexual reproduction generally, 
must be considered as ' a rejuvenescence of life.' Biitschli also 
accepts this view, at any rate as regards conj ugation. These author- 
ities imagine that the wonderful phenomena of life, of which 
the underlying cause is still an unsolved problem, cannot be con- 
tinued indefinitely by the action of forces arising from within 
itself, that the clock-work would be stopped after a longer or 
shorter time, that the reproduction of purely asexual organisms 
would cease, just as the life of the individual finally comes to an 
end, or as a spinning wheel comes to rest in consequence of friction, 
and requires a renewed impetus if its motion is to continue. In 
order that reproduction may continue without interruption, these 
writers believe that a rejuvenescence of the living substance is 
necessary, that the clock-work of reproduction must be wound up 
afresh ; and they recognize such a rejuvenescence in sexual repro- 
duction and in conjugation, or in other words in the fusion of two 
cells, whether in the form of germ-cells or of two unicellular 
organisms. 

Edouard van Beneden expresses this idea in the following words: — 
' II semble que la faculty que possedent les cellules, de se multiplier 
par division soit limitee : il arrive un moment oii elles ne sont plus 
capables de se diviser ulterieurement, k moins qu'elles ne subissent 
le phenomene du rajeunissement par le fait de la fecondation. 

' S. Hermann's ' Handbuch der Physiologie,' Theil II ; ' Physiologie der Zeugung,* 
by V. Hensen. 

' E. van Beneden, ' Recherches sur la maturation de Tceuf, la fecondation et la 
division cellulaire.* Gand u. Leipzig, 1 883, pp. 404 et seq. 



IN THE THEORY OF NATURAL SELECTION. 283 

Chez les animaux et les plantes les seules cellules capables d'etre 
rajeunies sont les ceufs ; les seules capables de rajeunir sont les 
spermatocytes. Toutes les autres parties de I'individu sont vouees 
k la mort. La fecondation est la condition de la continuite de la 
vie. Par elle le generateur echappe h la mort ' (1. c, p. 405). 
Victor Hensen thinks it possible that the germ and its products 
are prevented from dying by means of normal fertilization: he 
says that the law which states that every egg must be fertilized, 
was formulated before the discovery of parthenogenesis and cannot 
now be maintained, but that we are nevertheless compelled to 
assume that even the most completely partlienogenetic species 
requires fertilization after many generations (1. c, p. 336). 

If the theory of rejuvenescence be thoroughly examined, it will 
be found to be nothing more than the expression of the fact that 
sexual reproduction persists without any ascertainable limit. From 
the fact of its general occun-ence, the conclusion is, however, drawn 
that asexual reproduction could not persist indefinitely as the 
only mode of reproduction in any species of animal. But proofs 
in support of this opinion are wanting, and it is very probable 
that it would never have been advanced if it had been possible 
to explain the general occurrence of sexual reproduction in any 
other way, — if we had been able to ascribe any other significance 
to this pre-eminently important process. 

But quite apart from the fact that it is impossible to bring 
forward any proofs, the theory of rejuvenescence seems to me to 
be unsatisfactory in other ways. The whole conception of re- 
juvenescence, although very ingenious, has something uncertain 
about it, and can hardly be brought into accordance with the 
usual conception of life as based upon physical and mechanical 
forces. How can any one imagine that an Infusorian, which by 
continued division had lost its power of reproduction, could regain 
this power by forming a new individual, after fusion with another 
Infusorian, which had similarly become incapable of division? 
Twice nothing cannot make one. If indeed we could assume that 
each animal contained half the power necessary for reproduc- 
tion, then both together would certainly form an efiicient whole ; 
but it is hardly possible to apply the term rejuvenescence to a 
process which is simply an addition, such as would be attained 
under other circumstances by mere growth ; neg'lecting, for the 



284 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

present, that factor which, in my opinion, is of the utmost import- 
ance in conjugation, — the fusion of two hereditary tendencies. If 
rejuvenescence possesses any significance at all, it must be this, — 
that by its means a force, which did not previously exist in the 
conjugating individuals, is called into activity. Such a force 
would, however, owe its existence to latent energy stored up in 
each single animal during the period of asexual reproduction, and 
such latent forces would necessarily be of different natures, and of 
such a constitution that their union at the moment of conjugation 
would give rise to the active force of reproduction. 

The process might perhaps be compared to the flight of two 
rockets, which by the combustion of some explosive substance (such 
as nitro-glycerine) stored up within themselves are impelled in 
such a direction that they would meet at the end of their course, 
when all the nitro-glycerine had been completely exhausted. The 
movement would then come to an end, unless the explosive material 
could have been meanwhile renewed. Now suppose that such a 
renewal were achieved by the formation of nitric acid in one of the 
rockets and glycerine in the other, so that when they came into 
contact nitro-glycerine would be formed afresh equal in quantity and 
in distribution on both the rockets to that which was originally 
present. In this way the movement would be renewed again and 
again with the same velocity, and might continue for ever. 

Rejuvenescence can be rendered intelligible in theory by some 
such metaphor, but considerable difficulties are encountered in 
the rigid application of the metaphor to the facts of the case. 
In the first place, how is it possible that the motive force can be 
exhausted by continual division, while one of its components is 
being formed afresh in the same body and during the same time ? 
When thoroughly examined the loss of the power of division is 
seen to follow from the loss of the powers of assimilation, nutrition, 
and growth. How is it possible that such a power can be 
weakened and finally entirely lost while one of its components 
is accumulated? 

I believe that, instead of accepting such daring assumptions, it 
is better to be satisfied with the simple conception of living 
matter possessing as attributes the powers of unlimited assimilation 
and capacity for reproduction. With such a theory the mere form 
of reproduction, whether sexual or asexual, will have no influence 



IN THE THEORY OF NATURAL SELECTION. _„„ 

upoQ the duration of the capacity : for force and matter under^ 
simultaneous increase, and are inseparably" connected in this as 11 
all other instances. This theory does not, however, exclude the 
possible occurrence of circumstances under which such an associa- 
tion is no longer necessary. 

I could only consent to adopt the hypothesis of rejuvenescence, if 
it were rendered absolutely certain that reproduction by division 
could never under any circumstances persist indefinitely. But this 
cannot be proved with any greater certainty than the converse pro- 
position, and hence, as far as direct proof is concerned, the facts are 
equally uncertain on both sides. The hypothesis of rejuvenescence 
is, however, opposed by the fact of parthenogenesis ; for if fertilization 
possesses in any way the meaning of rejuvenescence, and depends 
upon the union of two difierent forms of force and of matter, which 
thus produce the power of reproduction, it follows that we cannot 
understand how it happens that the same power of reproduction 
may be sometimes produced from one form of matter, alone and 
unaided. Logically speaking, parthenogenesis should be as im- 
possible as that either nitric acid or glycerine should separately 
produce the effect of nitro-glycerine. The supposition has indeed 
been made that in the case of parthenogenesis, one fertilization is 
sufficient for a whole series of generations, but this supposition is 
not only incapable of proof, but it is contradicted by the fact that 
certain eggs which may develope parthenogenetically are also capable 
of fertilization. If, in this case, the power of reproduction were suf- 
ficient for development, how is it that the egg is also capable of 
fertilization ; and if the power were insufficient, how is it that the 
egg can develope parthenogenetically ? And yet one and the same 
egg (in the bee) can develope into a new individual, with or with- 
out fertilization. We cannot escape this dilemma by making 
the further assumption, which is also incapable of proof, that a 
smaller amount of reproductive force is required for the development 
of a male individual than for the development of a female. It is 
true that the unfertilized eggs of the bee produce male individuals, 
while the fertilized ones develope into females, but in certain other 
species the converse association holds good, while in others, again, 
fertilization bears no relation to the sex of the ofispring. 

Although the mere fact that parthenogenesis occurs at all is, in 
my opinion, sufficient to disprove the theory of rejuvenescence, it is 



286 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

w^U to remember that parthenog'enesis is now the only method of 
Reproduction in many species (althoug-h we do not know the period 
of time over which these conditions have extended), and is neverthe- 
less unattended by any perceptible decrease in fertility. 

From all these considerations we may draw the conclusion that 
the process of rejuvenescence, as described above, cannot be accepted 
either as the existing or the original meaning of conjugation, and 
the question haturally arises as to what other significance this 
latter process can have possessed at its first beginning. 

E/olph ^ has expressed the opinion that conjugation is a form of 
nutrition, so that the two conjugating individuals, as it were, devour 
each other. Cienkowsky^ also regards conjugation as merely 
accelerated ' assimilation. There is, however, not only an essential 
difference but a direct contrast between the processes of conjugation 
and nutrition. With regard to Cienkowsky's view, Hensen ^ has 
well said that ' coalescence in itself cannot be an accelerated nutrition, 
because even if we admit that both individuals are in want of 
nourishment, it is impossible that the need can be supplied by this 
process, unless one of them perishes and is really devoured.' In 
order that an animal may serve as the food of another, it must 
perish and must be brought into a fluid form, and finally it must be 
assimilated. In the case before us, however, two protoplasmic 
bodies are placed side by side and coalesce, without either of them 
passing into the liquid state. Two idioplasms unite, together with 
all the hereditary tendencies contained in them ; but although it is 
certain that nutrition in the proper sense of the word cannot take 
place, because neither of the animals receives an addition of liquid 
food by the coalescence, yet the consequence of this process must 
be in one respect similar to that of nutrition and growth : — the 
mass of the body and the quantity of the forces contained in it 
undergo simultaneous increase. It is not inconceivable that effects 
are by this means rendered possible, which under the peculiar 
circumstances leading to conjugation, could not have been otherwise 
produced. 

I believe that this is at any rate the direction in which we shall 
have to seek for the first meaning of conjugation and for its 

* Eolph, ' Biologische Probleme.' Leipzig, 1882. 

* Cienkowsky, 'Arch. f. inikr. Anat.,' ii. p. 47. 1873, 
' Hensen, ' Physiologie der Zeugung,' p. 1 39. 



IN THE THEOEY OF NATURAL SELECTION. 287 

phyletic orig-in. This first result and meaning of conjugation may 
be provisionally expressed in the following formula : — conjugation 
originally signified a strengthening of the organism in relation to 
reproduction, which happened when from some external cause, such 
as want of oxygen, warmth, or food, the growth of the individual 
to the extent necessary for reproduction could not take place. 

This explanation must not be regarded as equivalent to that 
afforded by the theory of rejuvenescence ; for the latter process is said 
to be necessary for the continuance of reproduction, and ought 
therefore to occur periodically quite independently of external cir- 
cumstances ; while according to my theory, conjugation at first 
only occurred under unfavourable conditions, and assisted the species 
to overcome such difficulties. 

But whatever the original meaning of conjugation may have 
been, it seems to have become already subordinated in the higher 
Protozoa, as is indicated by the changes in the course taken by 
this process. The higher Protozoa when conjugating- do not as a 
rule coalesce completely and permanently ^ in the manner followed 
by the lower Protozoa, and it seems to me possible, or even probable, 
that in the former the process has already gained the full significance 
of sexual reproduction, and is to be looked upon as a source of 
variability. 

Whether this be so or not, I believe it is certain that sexual re- 
production could not have been entirely abandoned at any period 
since the time when the Metazoa and Metaphyta first arose ; for they 
derived this form of reproduction from their unicellular ancestors. 

We know that organs and characters which have persisted 
through a long series of generations are transmitted with extreme 
tenacity, even when they have ceased to be of any direct use to 
their immediate possessors. The rudimentary organs in various 
animals, and not least in man, afford very strong proofs of the sound- 
ness of this conclusion. Another example has only recently been 
discovered in the sixth finger, which has been shown to exist in the 
human embryo ^, a part which has only been present in a rudimentary 

^ Coalescence takes place in the so-called bud-like conjugation of Vorticellidae and • 
Trichodinidae, etc. 

* Compare (i) Bardeleben, ' Zur Entwicklung der Fusswurzel,' Sitzungsber. d. Jen. 
Gesellschaft, Jahrg. 1885, Feb. 6; also ' Verbandl. d. Naturforscherversammlung 
zu Strassburg,' 1885, p. 203 ; (2) G. Baur, *Zur Morphologie des Carpus und Tarsus 
der Wirbelthiere,' Zool. Anzeiger, 1885, pp. 326, 486. 



288 THE SIGNIFICANCE OP SEXUAL REPRODUCTION 

form ever since the origin of the Amphibia ^. Superfluous organs 
become rudimentary very slowly, and enormous periods must elapse 
before they completely disappear, while the older a character is, 
the more firmly it becomes rooted in the organism. What I have 
above called the physical constitution of a species is based upon 
these facts, and upon them depend the tout ensemble of inherited 
characters, which are adapted to one another and woven together 
into a harmonious whole. It is this specific nature of an organism 
which causes it to respond to external influences in a manner different 
from that followed by any other organism, which prevents it from 
changing in any way except along certain definite lines of varia- 
tion, although these may be very numerous. Furthermore these 
facts ensure that characters cannot be taken at random from the 
constitution of a species and others substituted for them. Such a 
variation as a mammal wanting the firm axis of the backbone is an 
impossibility, not only because the backbone is necessary as a support 
to the body, but chiefly because this structure has been inherited 
from times immemorial, and has become so impressed upon the 
mammalian organization that any variation so great as to threaten 
its very existence cannot now take place. The view here set forth 
of the origin of hereditary variability by amphigonic reproduction, 
makes it clear that an organism is in a state of continual oscillation 
only upon the surface, so to speak, while the fundamental parts of 
its constitution, which have been inherited from extremely remote 
periods, remain unaffected. 

Thus sexual reproduction itself did not cease after it had existed 
in the form of conjugation through innumerable generations of the 
vast numbers of species which have been included under the Protozoa; 
it did not cease even when its original physiological significance 
had lost its importance, either completely or in part. This process, 
however, had come to possess a new significance which ensured its 
continuance, in the enormous advantage conferred on a species by 
the power of adapting itself to new conditions of life, a power which 
could only be preserved by means of this method of reproduction. 
The formation of new species which among the lower Protozoa 
could be achieved without amphigony, could only be attained by 
means of this process in the Metazoa and Metaphyta. It was only 

^ In frogs the sixth toe exists in the hind legs as a rudimentary prehallux. Com- 
pare Born, Morpholog. Jahrbuch, Bd. I, 1876. 



IN THE THEOKY OF NATUEAL SELECTION. 289 

in this way that hereditary individual differences could arise and 
persist. It was impossible for amphigony to disappear, for each 
species in which it was preserved was necessarily superior to those 
which had lost it, and must have replaced them in the course of 
time ; ^ for the former alone could adapt itself to the ever-chang-ing 
conditions of life, and the longer sexual reproduction endured, the 
more firmly was it necessarily impressed upon the constitution of 
the species, and the more difficult its disappearance became. 

Sexual reproduction has nevertheless been lost in some cases, 
although only at first in certain generations. Thus in the Aphidae 
and in many lower Crustacea, generations with parthenogenetic re- 
production alternate with others which reproduce themselves by the 
sexual method. But in most cases it is clear that this partial loss 
of amphigony conferred considerable advantages upon the species by 
giving increased capabilities for the maintenance of existence. By 
means of partial parthenogenesis a much more rapid increase in the 
number of individuals could be attained in a given time, and this 
fact is of the highest importance for the peculiar circumstances 
under which these species exist. A species of Crustacean which 
inhabits rapidly drying pools, and developes from winter-eggs which 
have remained dried up in the mud, has, as a rule, only a very 
short time in which to secure the existence of succeeding genera- 
tions. The few sexual eggs which have escaped the attacks of 
numerous enemies develope immediately after the first shower of 
rain ; the animals attain their full size in a few days and reproduce 
themselves as virgin females. Their descendants are propagated in 
the same manner, and thus in a short time almost incredible num- 
bers of individuals are formed, until finally the sexual eggs are 
again produced. If now the pool dries up again, the existence of 
the colony is secured, for the number of animals which produce 
sexual eggs is very large, and the eggs themselves are of course 
far more numerous, so that in spite of the destructive agencies to 
which they are subjected, there will be every chance of the survival 
of a sufficient number to produce a new generation at a later 
period. Here, therefore, sexual reproduction has not been abandoned 
accidentally or from any internal cause, but as an adaptation to 
certain definite necessities imposed upon the organism by its 
surroundings. 

It is, however, well known that there are certain instances in 

u 



290 THE SiaNIFICANCE OP SEXUAL REPRODUCTION 

which sexual reproduction has been altogether lost, and in which 
parthenogenesis is the only form of propagation. In the animal 
kingdom, such a condition chiefly occurs in species of which the 
closely-allied forms exhibit the above-mentioned alternation between 
parthenogenesis and amphigony, viz. in many Cynipidae and Aphid ae, 
and also in certain freshwater and marine Crustacea. We may 
imagine that these parthenogenetic species have arisen from forms 
with alternating methods of reproduction, by the disappearance of 
the sexual phase. 

In any particular case, it may be difficult to point out the motive 
by which this change has been determined ; but it is most probable 
that the same conditions which originally caused the intercalation 
of a parthenogenetic stage have been efficient in causing the 
gradual disappearance of the sexual stage. If a species of Crust- 
acean, with the above-described alternating method of reproduction 
(heterogeny), were killed off by its enemies on a larger scale than 
before, it is obvious that the threatened extinction of the species 
could be checked by the attainment of a correspondingly greater 
degree of fertility. Such increased fertility might well be produced 
by pure parthenogenesis (see Appendix V, p. ^1'^), by means of 
which the number of egg-producing individuals in all the previous 
sexual generations would be doubled. 

In a certain sense, this would be the last and most extreme 
method by means of which a species might secure continued 
existence, for it is a method for which it would have to pay 
very dearly at a later period. If my theory as to the causes of 
hereditary individual variability be correct, it follows that all species/ 
with purely parthenogenetic reproduction are sure to die out ; not< 
indeed, because of any failure in meeting the existing conditions 
of life, but because they are incapable of transforming themselves 
into new species, or, in fact, of adapting themselves to any new 
conditions. Such species can no longer be subject to the process 
of natural selection, because, with the disappearance of sexual re- 
production, they have also lost the power of combining and in- 
creasing those hereditary individual characters which they possess. 

All the facts with which we are acquainted confirm this con- 
clusion, for whole groups of purely parthenogenetic species or 
genera are never met with, as would certainly be the case if 
parthenogenesis had been the only method of reproduction through 



IN THE THEORY OF NATURAL SELECTION. 291 

a suecessional series of species. We always find it in isolated 
instances, and under conditions which compel the conclusion that 
it has become predominant in the species in question, and has not 
been transmitted from any preceding species. 

There still remains a very difierent class of facts which, so far as 
we can judg-e, are in accordance with my theory as to the signi- 
ficance of sexual reproduction, and which may be quoted in its 
support. I refer to the condition of functionless organs in species 
with parthenogenetie reproduction. 

Under the supposition that acquired characters cannot be trans- 
mitted — and this forms the foundation of the views here set forth 
— organs which are of no further use cannot become rudimentary 
in the direct and simple manner in which it has been hitherto 
imagined that degeneration takes place. It is true that an organ 
which does not perform any function exhibits a marked decrease of 
strength and perfection in the individual which possesses it, but 
such acquired degradation is not transmitted to its descendants, 
and we must therefore look for some other explanation of the 
firmly established fact that organs do become rudimentary through 
a series of generations. In seeking this explanation, we shall have 
to start from the supposition that new forms are not only created 
by natural selection, but are also preserved by its means. In 
order that any part of the body of an individual of any species may 
be kept at the maximum degree of development, it is necessary 
that all individuals possessing it in a less perfect form must be 
prevented from propagation — they must succumb in the struggle 
for existence. I will illustrate this by a special instance. In 
species which, like the birds of prey ^, depend for food upon the 
acuteness of their vision, all individuals with relatively weak eye- 
sight must be exterminated, because they will fail in the competition 
for food. Such birds will perish before they have reproduced them- 
selves, and their imperfect vision is not further transmitted. In 
this way the keen eyesight of birds of prey is kept up to its 
maximum. 

But as soon as an organ becomes useless, the continued selection 
of individuals in which it is best developed must cease, and 
a process which I have termed panmixia takes place. When this 

' I here make use of the same illustration which I employed in my first attempt 
to explain the effects oi panmixia. Compare the second Essay ' Qn Heredity.' 

u a 



292 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

process is in operation, not only those individuals with the best- 
developed organs have the chance of reproducing themselves, but 
also those individuals in which the organs are less well-developed. 
Hence follows a mixture of all possible degrees of perfection, 
which must in the course of time result in the deterioration of the 
avemge development of the organ. Thus a species which has retired 
into dark caverns must necessarily come to gradually possess less 
developed powers of vision ; for defects in the structure of the 
eyes, which occur in consequence of individual variability, are not 
eliminated by natural selection, but may be transmitted and fixed 
in the descendants^. This result is all the more likely to happen, 
inasmuch as other organs which are of importance for the life 
of the species will gain what the functionless organ loses in size 
and nutrition. As at each stage of retrogressive transformation 
individual fluctuations always occur, a continued decline from the 
original degree of development will inevitably, although very 
slowly, take place, until the last remnant finally disappears. How 
inconceivably slowly this process goes on is shown by the numerous 
cases of rudimentary organs : by the above-mentioned embryonic 
sixth finger of man, or by the hind limbs of whales buried beneath 
the surface of the body, or by their embryonic tooth-germs. 
I believe that the very slowness with which functionless organs 
gradually disappear, agrees much better with my theory than with 
the one which has been hitherto held. The result of the disuse of 
an organ is considerable, even in the course of a single individual 
life, and if only a small fraction of such a result were trans- 
mitted to the descendants, the organ would be necessarily reduced 
to a minimum, in a hundred or at any rate in a thousand genera- 
tions. But how many millions of generations may have elapsed 
since e. g. the teeth of the whalebone whales became useless, and 
were replaced by whalebone ! We do not know the actual number 
of years, but we know that the whole material of the tertiary rocks 
has been derived from the older sti*ata, deposited in the sea, elevated, 

[' E. Ray Lankester has suggested (Encycl. Britann., art. ' Zoology,' pp. 8i8, 819) 
that the blindness of cave-dwelling and deep-sea animals is also due to the fact that 
' those individuals with perfect eyes would follow the glimmer of light and eventually 
escape to the outer air or the shallower depths, leaving behind those with imperfect 
eyes to breed in the dark place. A natural selection would thus be efl'ected.' Such 
a sifting process would certainly greatly quicken the rate of degeneration due to pan- 
mixia alone.— E. B. P.] 



IN THE THEORY OP NATURAL SELECTION. 293 

and has been itself largely removed by denudation, since that 
time. 

Now if this theory as to the, causes of deterioration in disused 
organs be correct, it follows that rudimentary organs can only 
occur in species with sexual reproduction, and that they cannot be 
formed in species which are exclusively reproduced by the partheno- 
genetic method : for, according to my theory, variability depends ■ 
upon sexual reproduction, while the deterioration of an organ when 
disused, no less than its improvement when in use, depends upon 
variability. There are therefore two reasons which lead us to 
expect that organs which are no longer used will remain un-\ 
reduced in species with asexual reproduction : first, because only/ 
a very slight degree of hereditary variability can be present, viz. 
such a degree as was transmitted from the time when sexual 
reproduction was first abandoned by the ancestors ; and, secondly, 
because even these slight degrees of variability are not combined, 
or, in other words, because panmixia cannot occur. 

And the facts seem to point in the direction required by the 
theory, for superfluous organs do not become rudimentary in 
parthenogenetic species. For example, as far as my experience 
goes, the receptaculum seminu does not deteriorate, although it is, 
of course, altogether functionless when parthenogenesis has become 
established. I do not attach much importance to the fact that the 
Psychids and Solenobias — (genera of Lepidoptera which Siebold 
and Leuckart have shown to include species with parthenogenetic 
reproduction) — still retain the complete female sexual apparatus, 
because colonies containing males still occasionally occur in these 
species. Although the majority of colonies are now purely female, 
the occasional appearance of males points to the fact that the uni- 
sexuality of the majority cannot have been of very long duration. 
The process of transformation of the species from a bisexual into 
a unisexual form, only composed of females, is obviously in- 
complete, and is still in process of development. The case is 
similar with several species of Cynipidae, which reproduce by the 
parthenogenetic method. In these cases the occurrence of a very 
small proportion of males is the general rule, and is not confined to 
single colonies. Thus Adler^ counted 7 males and 664 females in 
the common Cynips of the rose. 

1 Adler, ' Zeitschrift f. wiss. Zool.,' Bd. XXXV, 1881. 



294 THE SiaNIFICANCE OF SEXUAL EEPRODUCTION 

In some Ostracodes, on the other hand, the males appear to be 
entirely wanting- : at least, I have tried in vain for years to 
discover them in any locality or at any time of the year^. 

Cyprii vidua and Cypn% reptans are such species. Now, although 
the transformation of these formerly bisexual species into purely 
unisexual female species appears to be complete^, yet the females 
still possess a large, pear-shaped receptaculum seminis, with its 
long spirally tvsdsted duct, which is surrounded by a thick glandular 
layer. This is the more remarkable as the apparatus is very 
complicated in the Ostracodes, and retrogressive changes could be 
therefore easily detected. Furthermore among insects, in the genus 
Ckermeg the receptaculum semiriis of the females has also remained 
unreduced, although the males appear to be entirely wanting, or 
at least have never been found, in spite of the united efforts of 
several acute observers^. The case is quite different in species which 
retain both sexual and parthenogenetic reproduction. Thvis, the 
summer females of the Aphiclae have lost the receptaculum seminis ; 
and in these insects sexual reproduction has not ceased, but alter- 
nates regularly with parthenogenetic reproduction. 

Certainly this proof of the truth of my theory as to tne signi- 
ficance of sexual reproduction is far from settling the question : it 
only renders the theory highly probable. At present it is im- 
possible to do more than this, because we do not yet possess 
a sufficient number of facts, for many of them could not have been 
sought for until after the theory had been suggested. We are here 
concerned with complicated phenomena, into which we cannot 
acquire an immediate insight, but can only attain it gradually. 

But, nevertheless, I hope to have shown that the theory of 

* Compare my paper, ' Parthenogenese bei den Ostracoden,' in ' Zool. Anzeiger,' 
1880, p. 82. Purely negative evidence, unless on an immense scale, is quite rightly 
considered to be of no great value in most cases. But the condition of these animals 
renders the accumulation of such evidence unusually easy, because the presence of 
males in a colony of Ostracodes can be proved by a very simple indirect test. Thus 
if a colony contains any males the receptacula seminis of all mature females are filled 
with spermatozoa, and on the other hand we may be quite sure that males are 
absent, if after the examination of many mature females, no spermatozoa can be 
found in any of their receptacula. 

^ We cannot, however, be absolutely certain of this, for it is conceivable that 
males may still occur in colonies other than those examined. 

^ It has now been shown by Blochmann that males appear for a very short time 
towards the close of summer, as in the case of Phylloxera. — A.W., 1888. 



IN THE THEORY OF NATURAL SELECTION. 295 

natural selection is by no means incompatible with the theory of 
' the continuity of the g-erm-plasm ; ' and, further, that if we 
accept this latter theory, sexual reproduction appears in an entirely 
new light : it has received a meaning-, and has to a certain extent 
become intelligible. 

The time in which men believed that science could be advanced 
by the mere collection of facts has long passed away : we know 
that it is not necessaiy to accumulate a vast number of miscel- 
laneous facts, or to make as it were a catalogue of them ; but we 
know that it is necessary to establish facts which, when grouped 
together iji the light of a theory, will enable us to acquire a certain 
degree of insight into some natural phenomenon. In order to 
direct our attention to those new facts which are of immediate 
importance, it is absolutely necessary to seek the aid of some 
general theory for the arrangement and grouping of those which 
we already possess. This has been my object in the present paper. 

But it may be perhaps objected that these phenomena are far too 
complicated to be attacked at the present time, and that we ought 
to wait quietly until the simpler phenomena have been resolved into 
their components. It may be asked whether the trouble and 
labour involved in the attempt to solve such questions as heredity 
or the transformation of species are not likely to be wasted and 
useless. 

It is true that we sometimes meet with such opinions, but I 
believe that they are based upon a misunderstanding of the method 
which mankind has always followed in the investigation of nature, 
and which must therefore be founded upon the necessary relations 
existing between mankind and nature. 

Science has often been compared to an edifice which has been 
solidly built by laying stone upon stone, until it has gradually 
risen to greater height and perfection. This comparison holds 
good up to a certain point, but it leads us to easily overlook the 
fact that this metaphorical building does not at any point rest 
upon the ground, and that, at least up to the present time, it has 
remained floating in the air. Not a single branch of science, not 
even Physics itself, has commenced building from below ; all 
branches have begun to build at greater or less heights in the air, 
and have then built downwards : and even Physics has not yet 
reached the ground, for it is still very uncertain as to the nature of 



296 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

matter and force. In no single group of phenomena can we begin 
with the investigation of ultimate causes, because at this very point 
our means of reasoning stop short. We cannot begin with ulti- 
mate phenomena and gradually lead up to those which are more 
complicated : we cannot proceed synthetically and deductively, 
building up the phenomena from below ; but we must as a rule 
proceed analytically and inductively, proceeding from above down- 
wards. 

No one will dispute these statements, but they are often for- 
gotten, as is proved by the above-mentioned objection. If we 
were only permitted to attack the more complicated phenomena 
after gaining a complete insight into the simpler ones, then all 
scientists would be physicists and chemists, and not until Physics 
and Chemistry were done with should we be permitted to proceed 
to the investigation of organic nature. Under these circumstances 
we ought not to possess now any scientific theory of medicine ; 
for the study of pathological physiology could not be commenced 
until normal physiology was completely known and understood. 
Yet how great a debt is owing by normal to pathological physio- 
logy! This is an example which enforces the conclusion that it 
is not only permissible, but in the highest degree advantageous, for 
the different spheres of phenomena to be attacked simultaneously. 

Furthermore, if we had been compelled to proceed from the 
simple to the complex, what would have become of the Theory of 
Descent, the influence of which has advanced our knowledge of 
Biology to an altogether immeasurable extent ? 

But in this often repeated criticism that we are not yet ready to 
attack such complicated phenomena as heredity, is hidden still 
another fallacy, for it is implied that facts become less certain in 
proportion to the complexity of their causes. But is it less certain 
that the egg of an eagle developes into an eagle, or that the pecu- 
liarities of the father and mother are transmitted to the child, than 
that a stone falls to the ground when its support is taken away ? 
Again, is it not possible to draw a perfectly distinct and certain 
conclusion as to the relative quantity of the material basis of 
heredity, present in the germ-cells of either parent, from the fact 
that the father and mother possess an equal or nearly equal share 
in heredity ? But it is really unnecessary to argue in this way : 
why should we do more than re-affirm that such a method of pro- 



IN THE THEOEY OF NATUEAL SELECTION. 297 

cedure in scientific investigation is the only way by which we can 
gradually penetrate the hidden depths of natural phenomena ? 

No ! Biology is not obliged to wait until Physics and Chemistry 
are completely finished ; nor have we to wait for the investigation • 
of the phenomena of heredity until the physiology of the cell is 
complete. Instead of comparing the progress of science to a build- 
ing, I should prefer to compare it to a mining operation, undertaken 
in order to open up a freely branching lode. Such a lode must 
not be attacked from one point alone, but from many points 
simultaneously. From some of these we should quickly reach the 
deep-seated parts of the lode, from others we should only reach its 
superficial parts ; but from every point some knowledge of the 
complex tout ensemhle of the lode would be gained. And the more 
numerous the points of attack, the more complete would be the 
knowledge acquired, for valuable insight will be obtained in every 
place where the work is carried on with discretion and perseverance. 

But discretion is indispensable for a fruitful result ; or, leaving 
our metaphor, facts must be connected together by theories, if 
science is to advance. Just as theories are valueless without a firm 
basis of facts, so the mere collection of facts, without relation and 
without coherence, is utterly valueless. Science is impossible with- 
out hypotheses and theories: they are the plummets with which 
we test the depth of the ocean of unknown phenomena, and thus 
determine the future course to be pursued on our voyage of dis- 
covery. They do not give us absolute knowledge, but they aflTord 
us as much insight as it is possible for us to gain at the present 
time. To go on investigating without the guidance of theories, 
is like attempting to walk in a thick mist without a track and 
without a compass. We should get somewhere under these circum- 
stances, but chance alone would determine whether we should reach 
a stony desert of unintelligible facts or a system of roads leading in 
some useful direction ; and in most cases chance would decide 
against us. 

In this sense I trust that the sign-post or compass which I offer 
may be accepted. Even though it should be its fate to be replaced 
by a better one at a later period, it will have fulfilled its object if 
it enables science to advance for even a short distance. 



298 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



APPENDICES. 

Appendix I. Fuether considerations which oppose Nageli's 

EXPLANATION OF TRANSFORMATION AS DUE TO INTERNAL CAUSES *. 

When I describe Nageli's theory of transformation as due to 
active causes lying within the organism, as a phyletic force of 
transformation, I do not mean to imply that it is one of those 
mysterious principles which, according to some writers, constitute 
the unconscious cause which directs the transformation of species. 
Nageli's idioplasm, which changes from within itself, is conceived as 
a thoroughly scientific, mechanically operating principle. This cause 
is undoubtedly capable of theoretical conception : the only question 
is whether it has any real existence. According to Nageli, the 
growing organic substance, the idioplasm, not only represents a 
perpetuum mobile rendered possible as long as its substance con- 
tinually receives from without the matter and force which are 
necessary for continuous growth, but it also represents a per- 
petuum variahile due to the action of internal causes ^. But this is 
just the doubtful point, viz., whether the structure of the idioplasm 
itself compels it to change gradually during the course of its growth, 
or whether it is not rather the external conditions which compel the 
ever slightly varying idioplasm to change in a Certain direction by 
the summation of small differences. It has been shown above that 
we do not gain anything by adopting Nageli's theory, because the 
main problem which organic nature offers for our solution, viz. 
adaptation, remains unsolved. Hence this theory does not explain 
the phenomena of nature, and I believe that there are also certain 
facts which are directly antagonistic to it. 

If the idioplasm really possessed the power of spontaneous varia- 
bility ascribed to it by Nageli ; if, as a result of its own growth, it 
were compelled to undergo gradual changes, and thus to produce 
new species, we should expect that the duration of species, genera, 

' Appendix to page 357. 
* I.e., p. 118. 



IN THE THEOKY OF NATURAL SELECTION. 299 

orders, &c. would be of approximately equal length respectively, at 
least in forms of equal structural complexity. The time required 
by the idioplasm to underg-o such changes as would constitute 
transformation into a new species ought to be always the same at 
equal heights in the scale of organization, that is, with equal com- 
plexity in the molecular structure of the idioplasm. It appears to 
me to be a necessary consequence of Nageli's theory that the causes 
of transformation lie solely in this molecular structure of the idio- 
plasm. If nothing more than a certain amount of growth, and 
consequently a certain period of time during which the organism 
reproduces itself with a certain intensity, is required to produce a 
change in the idioplasm, then we must conclude that the alteration 
in the latter must take place when this certain amount of growth 
has been reached, or after this certain period has elapsed. In other 
words, the time during which a species exists — from its origin as a 
modification of some older species, until its own transformation into 
a new one — must be the same in species with the same degree of 
organization. But the facts are very far ffom supporting this con- 
sequence of Nageli's theory. The duration of species is excessively 
variable : many arise and perish within the limits of a single 
geological formation, while others may be restricted to a very small 
part of a formation ; others again may last through several forma- 
tions. It must be admitted that we cannot estimate the exact 
position of extinct species in the scale of organization, and the 
differences may therefore depend upon differences of organization : 
or they may be explained by the supposition that certain species 
may have become incapable of transformation, and might, under 
favourable conditions, continue to exist for an indefinite period. 
But this reply would introduce a new hypothesis in direct anta- 
gonism to Nageli's theory, which assumes that the variability of 
idioplasm takes place as the consequence of mere growth, and ne- 
cessarily depends upon molecular structure. Nageli himself asserts 
that the essential substance (idioplasm) of the descendants of the 
earliest forms of life is in a state of perpetual change, which would 
continue even if the series of successive generations were indefinitely 
prolonged ^. Hence there can be no rest in the process of change 
which the idioplasm must undergo ; and this is as true of each 
single species as it is of the organic world taken as a whole. We 

1 1. c, p. 1 1 8. 



300 THE SIGNIFICANCE OF SEXUAL REPEODQCTION 

could, perhaps, find shelter in the insufficiency of our geological 
knowledge, but the number of ascertained facts is too great for this 
to be possible. Thus it is well known that the genus Nautilus has 
lasted from Silurian times, through all the three geological periods, 
up to the present day; while all its Silurian allies (OrtAoceras, 
GompAoceras, Goniatites, &c.) became extinct at a comparatively 
early period. 

A keen and clever controversialist might still bring forward 
many objections against such an argument. I do not therefore 
place too much dependence upon the geological facts by themselves, 
as a disproof of the sel f- variability of Nageli's idioplasm ; for it must 
be admitted that the facts are not sufficiently complete for this 
purpose. For instance, in the case of Nautilus it might be argued 
that we do not know anything about the fossil Cephalopods of 
pre- Silurian times, and that it is therefore possible that the above- 
mentioned allies of Nautikis may have existed previously for as 
long a period as that through which Nautilus has lived in post- 
Silurian time. However this may be, it will be at least conceded 
that the geological facts do not lend any support to Nageli's 
theory, for we can see no trace of even an approximately regular 
succession of forms. 



Appendix II. Nageli's explanation of adaptation ^ 

In order to explain adaptation Niigeli assumes that, under certain 
circumstances, external influences may cause slight permanent 
changes in the idioplasm. If then such influences act continually 
in the same direction during long periods of time, the changes in 
the idioplasm may increase to a perceptible amount, i. e. to a degree 
which makes itself felt in visible external characters ^. But such 
changes alone could not be considered as adaptations, for the essen- 
tial character of an adaptation is that it must be a purposeful 
change. Nageli, however, brings forward the fact that external 
stimuli often produce their chief effects at that very part of the 
organism to which the stimuli themselves were applied. ' If the 
results are detrimental, the organism* attempts to defend itself 
against the stimulus: a confluence of nutrient fluid takes place 
towards the part upon which the stimulus has acted, and new tissues 

* Appendix to psge 258. * 1. c, p. 137. 



IN THE THEORY OF NATURAL SELECTION. 301 

are formed which restore the integrity of the org-anism by replacing 
the lost structures as far as- possible. Thus in plants the healthy 
tissues begin to grow actively around the seat of an injury, tending 
to close it up, and to afford protection by impenetrable layers of 
cork.' Purposeful reactions of this kind are certainly common in 
the organic world, occurring in animals as well as in plants. Thus 
in the human body an injury causes a rapid growth of the surround- 
ing tissues, which leads to the closing-up of the wound ; while in the 
Salamander even the amputated leg or tail is replaced by growth. 
An extreme example of these purposeful reactions is afforded by 
the tree-frog (Hi/la), which is of a light-green colour when seated 
upon a light-green leaf, but becomes dark brown when transferred 
to dark surroundings. Hence this animal adapts itself to the colour 
of its environment, and thus gains protection from its enemies. 

Admitting this capability on the part of organisms to react under 
certain stimuli in a purposeful manner, the question remains 
whether such a power is a primitive original quality belonging to 
the essential nature of each organism. The power of changing the 
colour of the skin in correspondence with that of the surroundings 
is not very common in the animal kingdom. In the frog this 
power depends upon a highly complex reflex mechanism. Certain 
chromatophores in the skin are connected with nerves ^ which pass 
to the brain and are there brought into relation, by means of nerve- 
cells, with the nervous centres of the organ of vision. The relation 
is of such a kind that strong light falling upon the retina consti- 
tutes a stimulus for the production of an impulse, which is conducted, 
along the previously mentioned motor nerves, from the brain to 
the chromatophores, thus determining the contraction of these 
latter and the consequent appearance of a light-coloured skin. 
When the strong stimulus (of light) ceases, the chromatophores 
expand again, and the skin becomes dark. That the chromato- 
phores do not themselves react upon the direct stimulus of light 
was proved by Lister 2, who showed that blind frogs do not possess 
the power of altering their colour in correspondence with that of 
their environment. It is quite obvious that in this case we are not 
dealing with a primary, but with a secondarily produced character ; 

^ Compare Briicke, ' Farbenwechsel des Chamaleon.' Wien. Sitzber. 1851. Also 
Ley dig, 'Die in Deutschland lebenden Saurier,' 1872. 
^ ' Philosophical Transactions,' vol. cxlviii. 1858, pp. 627-644. 



302 THE SiaNIPICANCE OP SEXUAL REPRODUCTION 

and it has yet to be proved that all the purposeful reactions men- 
tioned by Nageli are not similarly secondary characters or adapta- 
tions, and thus very far from being primitive qualities of the 
organic substance of the forms in which they occur. 

I do not by any means doubt that some of the reactions wit- 
nessed in organisms do not depend upon adaptation, but such 
reactions are not usually purposeful. Curiously enough, Nageli 
mentions the formation of galls in plants among his instances of 
purposeful reactions under external stimuli. I think, however, 
that it can hardly be maintained that the galls are of any use to 
the plant : on the contrary, they may even be very injurious to it. 
The gall is only useful to the insect which it protects and supplies 
with food. The recent and most excellent investigations of Adler * 
and of Beyerinck ^ have shown that the puncture made by the 
Cynips in depositing its eggs is not the stimulus which produces 
the gall, as was formerly believed to be the case, but that such a 
stimulus is provided by the larva which developes from the ^%^. 
The presence of this small, actively moving, foreign body stimu- 
lates the tissue of the plant in a definite manner, always producing 
a result which is advantageous to the larva and not to the plant. 
It would be to the advantage of the latter if it killed the in- 
truding larva, either enclosing it by woody tissue devoid of nourish- 
ment, or poisoning it by some acrid secretion, or simply crushing 
it by the active growth of the surrounding tissues. But nothing of 
the kind occurs : in fact an active growth of cells (forming the 
so-called 'Blastem' of Beyerinck) takes place around the embryo, 
while it is still enclosed in the egg-capsule ; but the growth is not 
such as to crush the embryo, which remains free in the cavity, the 
so-called larval chamber, which is formed around it. It would be 
out of place to discuss here the question as to how we can conceive 
that the plant is thus compelled to produce a growth which is at 
any rate indifferent and may be injurious to it ; and which, more- 
over, is exactly adapted to the needs of its insect-enemy. But it 
is at all events obvious that this cannot be an example of a self- 



• Adler, ' Beitrage zur Naturgeschichte der Cyrdpiden,' Deutsche entom. Zeitschr. 
XXI., 1877, p. 209; and by the same author, ' Ueber den Generationswechsel der 
Eichen-Gallwespen,' Zeitschr. f. wiss. Zool., Bd. XXXV. 1880, p. 151. 

* Beyerinck, ' Beobachtungen iiber die ersten Entwicklungsphasen einiger Cy- 
nipidengallen/ Verhandl. d. Amsterd. Akad. d. Wiss. Bd. XXII. 1883. 



IN THE THEORY OF NATURAL SELECTION. 303 

protecting" reaction under a stimulus, and that therefore an org-anism 
does not always respond to external stimuli in a manner useful to 
itself. 

But even if we could accept the suggestion that the purposeful 
reaction of an organism under stimulation is a primary and not a 
secondarily produced character, such a principle would by no means 
suffice for the explanation of existing adaptations. Nageli attempts 
to explain certain selected cases of adaptation as the direct results 
of external stimuli. He looks upon the thick hairy coat of mam- 
mals in arctic regions, and the winter covering of animals in tem- 
perate regions, as a direct reaction of the skin under the influence 
of cold. He considers that the horns, claws, and tusks of animals 
have arisen directly as reactions under stimuli applied to certain 
parts of the surface of the body in attack and defence ^. This inter- 
pretation is similar to that offered by Lamarck at the beginning of 
this century. At first sight such a suggestion appears to be 
plausible, for the acquisition of a thick hairy covering by the 
mammals of temperate regions is actually contemporaneous with 
the cold season of the year. But the question arises as to whether 
the production of a larger number of hairs at the beginning of 
winter is not merely another instance of a secondary character, like 
the assumption of a green colour by the tree-frog under the stimulus 
exerted by strong light. 

In the case of the hairy coat it is only necessary to produce a 
larger number of structures such as had existed previously ; but how 
can* it have been possible for the petals of flowers, with their 
peculiar and complex forms, to have been developed from stamens 
as a direct result of the insects which visit them in order to obtain 
pollen and nectar ? How could the creeping* of these insects and 
the small punctures made by them constitute stimuli for the produc- 
tion of an increased rate of growth ? And how is it possible in any 
way to explain, by mere increase in growth, the origin of a struc- 
ture in which each part has its own distinct meaning and plays 
a peculiar part in attracting insects and in the process of cross- 
fertilization effected by them ? Even if the manifold peculiarities of 
form could be explained in this way, how can such an explanation 
possibly hold for the colours of flowers? How could the white 
colour of flowers which open at night be explained as the direct 

1 1. c, p. 144. 



304 THE SIGNIPICANCE OF SEXUAL REPRODUCTION 

result of the creeping of insects ? How can the suggestion of such 
a cause offer any interpretation of the fact that flowers which open 
by day are tinted with various colours, or of the fact that there is 
often a bright or highly coloured spot which shows the way to the 
hidden nectary ? 

There are, moreover, a large number of very striking adaptations 
in form and colour, for which no stimulus acting directly upon the 
organism can be found. Can we imagine that the green caterpillar^, 
plant-bug, or grasshopper, sitting among green surroundings, 
is thus exposed to a, stimulus which directly produces the green 
colour in the skin ? Can the walking-stick insect, which resembles 
a brown twig, be subject to a transforming stimulus by sitting on 
such branches or by looking at them ? Or again, if we consider 
the phenomena of mimicry, how can one species of butterfly, by 
flying about with another species, exercise upon the latter such an 
influence as to render it similar to the first in appearance ? In 
many cases of mimicry, the mimicked and the mimicking species 
do not even live in the same place, as we see in the moths, flies, 
and beetles which resemble in appeai'ance the much-dreaded wasps. 

The interpretation of adaptation is the weak part of Nageli's 
theory, and it is somewhat remarkable that so acute a thinker 
should not have perceived this himself. One very nearly gains 
the impression that Nageli does not wish to understand the theory 
of natural selection. He says, for instance, in speaking of the 
mutual adaptation observable between the proboscis, the so-called 
* tongue ' of butterflies, and flowers with tubular corolla^ : — ' Among 
the most remarkable and commonest adaptations observable in the 
forms of flowers, are the corollas with long tubes considered in re- 
lation to the long " tongues " of insects, which suck the nectar from 
the bottom of the long narrow tubes, and at the same time effect 
the cross-fertilization of the plant. Both these arrangements have 
been gradually developed to their present degree of complexity — 
the long-tubed corollas from those without tubes, and from those 

[* It 18 now known that many such caterpillars are actually modified in colour by 
their surroundings, but the process appears to be indirect and secondarily acquired by 
the operation of natural selection, like that of the change of colour in the chamaeleon, 
fiivgs, fish, etc. ; although the stimulus of light acts upon the eyes of the latter animals 
and upon the skin of the caterpillar. See the seventh Essay (pp. 394-397) for a more 
detailed account. — E. B. P.] 

* I c, p. 150. 



IN THE THEORY OF NATURAL SELECTION. 305 

with short ones, the long " tongues " from short ones. Undoubtedly 
both have been developed at the same rate so that the length of 
both sets of structures has always remained the same.' 

No objection can be raised against these statements, but Nageli 
goes on to say : — ' But how can such a process of development be 
explained by the theory of natural selection, for at each stage in 
the process the adaptation was invariably complete. The tube of 
the corolla and the " tongue " must have reached, for instance, at 
a certain time, a length of 5 or lo mm. If now the tube of the 
corolla became longer in some plants, such an al<lL-ration would have 
been disadvantageous because the insects would be no longer able to 
obtain food from them, and would therefore visit flowers with 
shorter tubes. Hence, according to the theory of natural selection, 
the longer tubes ought to have disappeared. If on the other hand 
the " tongue " became longer in some insects, such a change would be 
superfluous and should have been given up, according to the same 
theory, as unnecessary structural waste. The simultaneous change 
in the two structures must, according to the theory of natural 
selection, be due to the same principle as that by which Miinch- 
hausen pulled himself out of a bog by means of his own pig-tail.' 

But, according to the theory of natural selection, the case appears 
in a very different light from that in which it is put by Nageli. 
The flower and the insect do not compete for the greater length of 
their respective organs : all through the gradual process, the flower 
is the first to lengthen its corolla and the butterfly follows. Their 
relation is not like that between a certain species of animal and 
another which serves as its prey, where each strives to be the 
quicker, so that the speed of both is increased to the greatest possible 
extent in the course of generations. Nor do they stand in the 
same relation as that obtaining between an. insectivorous bird and a 
certain species of butterfly which forms its principal food ; in such 
a case two totally different characters may be continually increased 
up to their highest point, e.g. in the butterfly similarity to the 
dead and fallen leaves among which it seeks protection when 
pursued, in the bird keenness of sight. As long as the latter 
quality is still capable of increase, so long will it still be advanta- 
geous to any individual butterfly to resemble the leaf a little more 
completely than other individuals of the same species ; for it will 
thus be capable of escaping those birds which possess a rather 

X 



306 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

keener sight than others. On the other hand, a bird with rather 
keener sight will have the greatest chance of catching the better 
protected butterflies. It is only in this way that we can explain 
the constant production of such extraordinary similarities between 
insects and leaves or other parts of plants. At every stage of 
growth both the insect and its pursuer are completely adapted to 
each other ; i. e. they are so far protected and so far successful 
respectively, as is necessary to prevent that gradual decrease in the 
av^erage number of individuals which would lead to the extermina- 
tion of the species ^. But the fact that there is complete adaptation 
at each stage does not prevent the two species from increasing 
those qualities of protection and of pursuit upon which they respec- 
tively depend. So far from this being the case, they would be 
necessarily compelled to gradually increase these qualities so long 
as the physical possibility of improvement remained on both sides. 
As long as some birds possessed a rather keener sight than those 
which previously existed, so long would those butterflies possess an 
advantage in which the resemblance to leaf-veining was more dis- 
tinct than in others. But from the moment at which the maximum 
keenness of eyesight attainable had been reached, at which there- 
fore all butterflies resembled leaves so completely that even the 
birds with the keenest eyesight might fail to detect them when at 
rest, — from this very point any further improvement in the simi- 
larity to leaves would cease, because the advantage to be gained from 
any such improvement would cease at the same time. 

Such reciprocal intensification of adaptive characters appears to 
me to have been one of the most important factors in the transfor- 
mation of species : it must have persisted through long series of 
species during phylogeny: it must have affected the most diverse 
parts and characters in the most diverse groups of organisms. 

In certain large butterflies of the Indian and African forests 
— Kallima paraUcta, K. inacliu, and K. alhofasciata — it has been 
frequently pointed out that the deceptive resemblance to a leaf is so 
striking that an observer who has received no hint upon the subject 
believes that he sees a leaf, even when he is looking at the butter- 
fly very closely. The similarity is nevertheless incomplete ; for out 

' In order to make the case as simple as possible, I assume that the insectivorous 
bird feeds upon a single species of insect, and that the insect is only attacked by a 
single species of bird. 



IN THE THEORY OF NATURAL SELECTION. 307 

of sixteen specimens in the collections at Amsterdam and Leyden, 
I could not find a sing-le one which had more than two lateral veins 
on one side of the mid-rib of the supposed leaf, or more than three 
upon the other side ; while about six or seven veins should have 
been present on each side. But from two to three lateral veins are 
amply sufficient to produce a hig-h degree of resemblance ; in fact 
so much so that it is a matter for wonder as to how it has been 
possible for such a relatively perfect copy to have been produced ; 
or how the sight of birds can have become so highly developed that 
while flying rapidly they could perceive the vein-like markings ; or 
to state the case more accurately, that they could detect those indi- 
viduals with a less number of veins than others. It is possible that 
the process of increase in resemblance is still proceeding in the 
species of the genus Kallima ; at all events, I was struck by the 
rather strong individual differences in the markings of the supposed 
leaf. 

On the other hand, the cause of the increase in length of the 
tubular corolla and of the butterfly's 'tongue,' lies neither in the 
flower nor in the butterfly, but it is to be found in those other 
insects which visit the flower and steal its honey without being of 
any assistance in cross-fertilization. It may be stated shortly, that 
non-tubular corollas, with the honey freely exposed — for it must be 
assumed the ancestral form was of this kind — gradually developed 
into corollas with the honey deeply concealed. The whole process 
was presumably first started by the flower, for the gradual with- 
drawal of the honey to greater depths conferred the advantage of 
protection from rain (Hermann Miiller), while larger quantities of 
honey could be stored up, and this would also increase the num- 
ber of insects visiting the flower and render their visits more 
certain. As soon as this withdrawal occurred, the mouth-parts of 
insects began to be subjected to a selective process whereby these 
organs in some of them were lengthened at the same rate as that at 
which the honey was withdrawn. When once the process had 
begun, its continuance was ensured, for as soon as flower-frequent- 
ing insects were divided into two groups with short and with long 
mouth-parts respectively, a further increase in the length of the 
corolla-tube necessarily took place in all those flowers which were 
especially benefited by the assured visits of a relatively small 
number of species of insects, viz., those flowers in which cross- 

X 2 



30& THE SIGNIPICANCE OP SEXUAL EEPRODUCTION 

fertilization was more certainly performed in this way than by the 
uncertain visits of a great variety of species. This would imply 
that a still further increase in leng-th would take place, for it is 
obvious that the cross-fertilization of any flower would be more cer- 
tainly performed by an insect when the number of species of plants 
visited by it became less ; and hence the cross-fertilization would 
be rendered most certain when the insect became completely 
adapted — in size, form, character of its surface, and the manner in 
which it obtained the honey — to the peculiarities of the flower. 
Those insects which obtain honey from a great variety of flowers 
are sure to waste a great part of the pollen by carrying it to the 
flowers of many different species, while insects which can only 
obtain honey from a few species of plants must necessarily visit 
many flowers of the same species one after the other, apd they 
would therefore more generally distribute the pollen in an effective 
manner. 

Hence the tube of the corolla, and the ' tongue ' of the butterfly 
which brings about fertilization, would have continued to increase 
in length as long as it remained advantageous for the flower to ex- 
clude other less useful visitors, and as long as it was advantageous 
for the butterfly to secure the sole possession of the flower. Hence 
there is no competition between the flower and the butterfly which 
fertilizes it, but between these two on the one side, and the other 
would-be visitors of the flower on the other. Fui-ther details as to 
the advantages which the flower gains by excluding all other 
visitors, and the butterfly by being the only visitor of the flower, 
and also as to the manifold and elaborate mutual adaptations between 
insects and flowers, and as to the advantages and disadvantages 
which follow from the concealment of the honey — will be found in 
Hermann Miiller's ^ work on the fertilization of flowers, in which 
all these subjects are minutely discussed, and are clearly explained 
in a most admirable manner. 

Appendix III. Adaptations in Plants 2. 

It is well known that Christian Conrad Sprengel was the first 
to recognise that the forms and colours of flowers are not due to 

• English Edition, translated by D'Aa-cy W. Thompson, B.A. London, 1883, 
p. 509 et seqq. 

" Appendix to page 260. 



IN THE THEORY OF NATURAL SELECTION. 309 

chance, that they are not the mere sport, bf nature, and that they 
are not made for the enjoyment of man, but that their purpose 
is to attract insects for the performance of cross-fertilization. It 
is also well known that this discovery — which was made at the 
end of the last century, and which caused much excitement at that 
time — was completely forgotten, and was brought to light again by 
Charles Darwin when attacking the same problem. 

In his work entitled 'The Solution of Nature's Secret in the 
Structure and Fertilization of Flowers ' (' Das entdeckte Geheimniss 
der Natur im Bau und der Befruchtung der Blumen'), published at 
Berlin, in 1793, Sprengel showed, in several hundred cases, that the 
peculiarities in the structure and colours of flowers were calculated 
to attract insects, and to ensure the fertilization of the flowers by 
their instrumentality. But it was due to his successor in this line 
of investigation that the whole significance of the cross-fertilization 
efiected by insects was made clear. Darwin ^ showed that in many 
cases, although not in all, the intention of nature was to avoid 
self-fertilization, and he showed that stronger and more numerous 
descendants are produced after cross-fertilization. 

After Darwin, several investigators, such as Kemer, Delpino 
and Hildebrand, have paid further attention to the subject, but it 
has been especially studied in a most thorough manner by Her- 
mann Miiller 2, He looked at the subject from more than one point 
of view, and showed by direct observation the species of insects 
which efiect cross-fertilization in various species of our native 
flowers : he also studied the structure of insects in relation to 
that of flowers, and attempted to establish the mutual adapta- 
tions which exist between them. In this way he succeeded in 
throwing muth light upon the process of transformation in many 
species of flowers, and in proving that certain insects, although un- 
consciously, are, as it were, breeders of certain forms of flowers. He 
not only distinguished the disagreeably smelling, generally in- 
conspicuous flowers (' Ekelblumen ') produced by Diptera which live 
on putrid substances, and the flowers which are produced by butter- 
flies ; but he also distinguished the flowers bred by saw-flies, by 

^ Ch. Darwin, 'On the fertilization of Orchids by Insects.' London, 1877. 

^ Compare Hermann Miiller, ' Die Befruchtung der Blumen durch Insekten und 
die gegenseitigen Anpassungen beider.' Leipzig, 1873. See also many articles by the 
same author in * Kosmos,' and other periodicals. These later articles are included 
in the English translation by D'Arcy W, Thompson. 



I 



310 THE SIONIFICANCE OP SEXUAL REPRODUCTION 

Fossoria, and by bees. He even believes that in certain cases ( Viola 
calcarafa) he can prove that a flower which owed its original form to 
being bred by bees, was afterwards adapted to cross-fertilization by 
butterflies, when it had migrated into an Alpine region where the 
latter insects are far more abundant than the former. 

Although there must of course be much that is hypothetical in 
the interpretations of the different parts of flowers offered by 
Hermann Miiller, the majority of these explanations are certainly 
correct, and it is of the greatest interest to be able to recognise the 
adaptive character of details, even when apparently tmimportant, 
in the structure and colours of flowers. 

Sachs has offered a very convincing explanation as to the mean- 
ing of leaf-veining, and of its significance in relation to the 
functions of leaves^. He shows that the venation of a leaf is in 
every case exactly adapted for the fulfilment of its purpose. It 
has, in the first place, to conduct the nutrient fluid in both direc- 
tions, and in the second place to support the thin layers of assimi- 
lating chlorophyll cells, and to stretch them out so as to expose as 
large a surface as possible to the light ; lastly, it has to toughen 
the leaf as a protection against being torn. He shows in a very 
convincing manner that the whole diversity of leaf venation can be 
understood from these three principles. Here, again, we meet with 
purposeful arrangements in a class of structures in which it was 
formerly thought that there was only a chaos of accidental forms, 
or, as it were, the mere sport of nature with form. 

Appendix IV. On the supposed transmission of acquired 

CHARACTERS 2. 

I 

When I previously maintained that the proofs of the trans- 
mission of artificially produced diseases are inconclusive, I had in 
mind the only experiments which, as far as I am aware, can be 
adduced in favour of the transmission of acquired characters; viz. 
the experiments of Brown-Sequard ^ on guinea-pigs. It is well 

* ' Lectures on the Physiology of Plants,' translated by H. Marshall Ward, 
Oxford, 1887, p. 47. 

^ Appendix to page 267. 

* Brown-Sequard, 'Researches on epilepsy; its artificial production in animals 
and its etiology, nature, and treatment.' Boston, 1857. Also various papers by the 
same author in 'Journal de physiologic de I'homme,' Tome I and III, 1858, i860, 
and in 'Archives de physiologic normale et pathologique,' Tome I-IV, 1868-1872. 



IN THE THEORY OF NATURAL SELECTION. 311 

known that' he produced artificial epilepsy in these animals by 
dividing certain parts of the central and also the peripheral nervous 
system. The descendants of the animals which acquired epilepsy 
sometimes inherited the disease of their parents. 

These experiments have been since repeated by Obersteiner ^, 
who has described them in a very exact and entirely unprejudiced 
manner. The fact itself cannot be doubted : it is certain that some 
of the descendants of animals in which epilepsy has been artificially 
produced, have also themselves sufiered from epilepsy in conse- 
quence of the disease of their parents. This fact may be accepted 
as proved, but in my opinion we have no right to conclude from it 
that acquired characters can be transmitted. Epilepsy is not 
a morphological character ; it is a disease. We could only speak 
of the transmission of a morphological character, if a certain mor- 
phological change which was the cause of epilepsy had been pro- 
duced by the nervous lesion, and if a similar change had re-appeared 
in the offspring, and had produced in them also the symptoms of 
epilepsy. But that this really occurs is utterly unproved ; and is 
even highly improbable. It has only been proved that many de- 
scendants of artificially epileptic parents are small, weakly, and very 
soon die ; and that others are paralysed in various parts of the 
body, i. e. in one or both of the posterior or anterior extremities ; 
while others again exhibit trophic paralysis of the cornea leading 
to inflammation and the formation of pus. In addition to these 
symptoms, the descendants in very rare cases exhibit upon the 
application of certain stimuli to the skin, a tendency towards those 
tonic and clonic convulsions together with loss of consciousness 
which constitute the features of an epileptic attack. Out of thirty- 
two descendants of epileptic parents only two exhibited such symp- 
toms, both of them being very weakly, and dying at an early age. 

These experiments, although very interesting, do not enable us 
to assert that a distinct morphological change is transmitted to 
the offspring after having been artificially induced in the parents. 
The injury caused by the division of a nerve is not transmitted, 
and the part of the brain corresponding to that which was removed 
from the parent is not absent from the offspring. The symptoms of 
a disease are undoubtedly transmitted, but the cause of the disease 
in the offspring is the real question which requires solution. The 

^ ' OesteiTeichische medicinische Jahrbiicher.' Jahrgang, 1875, p. 179. 



313 THE SIGNIFICANCE OP SEXUAL REPRODUCTION 

symptoms of epilepsy are by no means invariably transmitted; 
they are in fact absent from the great majority of cases, and the 
very small proportion in which they do occur, exhibit the symptoms 
of other diseases in addition to those of epilepsy. The offspring- 
are either quite healthy (thirteen out of thirty cases) or they suffer 
from disturbances of the nervous system, such as the above- 
mentioned motor and trophic paralysis, — symptoms which are not 
characteristic of epilepsy: however in some of the latter epilepsy 
is also present. 

If therefore we wish to express the matter correctly we must 
not state that epilepsy is transmitted to the offspring", but we must 
express the facts in the following manner : — animals which have 
been rendered epileptic by artificial means, transmit to some of their 
offspring a tendency to suffer from various nervous diseases, viz. 
from motor paralysis, to a less degree from sensory, and to a high 
degree from trophic paralysis ; in rare cases, when the symptoms 
of paralysis are very marked, epilepsy is also transmitted. 

If we now remember that a considerable number of diseases are 
already known to be caused by the presence of living organisms 
in the body, and that these diseases may be transmitted from one 
organism to another in the form of germs, ought we not to con- 
clude from the above-mentioned facts, that the symptoms are due 
to an unknown microbe which finds its nutritive medium in the 
nervous tissues, rather than to suppose that they are due to 
morphological changes, such as a modification of the histological 
or molecular structure of certain parts of the nervous system? 
At all events, it would be more difficult to understand the trans- 
mission of such a structural change, than the passage of a bacillus 
into the sperm- or germ-cell of the parent. There is no ascertained 
fact which supports the former assumption, but it is very probable 
that the transmission of syphilis, small-pox and tuberculosis^ is to be 
explained by the latter method, although the bacilli have not yet 
been detected in the reproductive cells. Furthermore, this method 
of transmission has been rigidly proved in the case of the mus- 

* A direct transmission of the germs of disease through the reproductive cells 
has lately been rendered probable in the case of tuberculosis, for the bacilli have 
been found in tubercles in the lungs of an eight- months' foetal calf, the mother being 
affected at the time with acute tuberculosis. However it is not impossible that 
infection may have arisen through the placenta. See ' Fortschritte der Medicin,' 
Bd. Ill, 1885, p. 198. 



IN THE THEORY OP NATURAL SELECTION. 313 

cardine disease of the silkworm. At all events we can understand 
in this way how it happened that the offspring- of artificially 
epileptic guinea-pig's were affected with various forms of nervous 
disease, a fact which would be quite unintelligible if we assume 
the occurrence of a true hereditary transmission of a morpho- 
logical character, such as a pathological change in the structure of 
some nervous centre. 

The manner in which artificial epilepsy becomes manifest after 
the operation, is also in favour of the explanation offered above. 
In the first place epilepsy does not result from any one single 
injury to the nervous system, but it may follow from a variety 
of different injuries. Brown-Sequard produced it by removing 
a portion of the grey matter of the brain, and by dividing 
the spinal cord, although the disease also resulted from a trans- 
verse section through half of the latter organ, or from the section of 
its anterior or posterior columns alone, or from simply puncturing 
its substance. The most striking effects appeared to follow when 
the spinal cord was injured in the region between the eighth 
dorsal and the second lumbar vertebrae, although the results were 
sometimes also produced by the injury of other parts. Epilepsy 
also followed the division of the sciatic nerve, the internal popliteal, 
and the posterior roots of all nerves which pass to the legs. The 
disease never appears at once, but only after the lapse of some 
days or weeks, and, according to Brown-Sequard, it is impossible 
to conclude that the disease will not follow the operation until 
after six or eight weeks have passed without an epileptic attack. 
Obersteiner did not witness in any case the first symptoms of the 
disease for several days after the division of the sciatic nerve. 
After the operation, sensibility decreases over a certain area on 
the head and neck, on the same side as the injury. If the animal 
be pinched in this region (which is called the epileptic area, ' zone 
epileptogene ') it curves itself round towards the injured side, and 
violent scratching movements are made with the hind leg of 
the same side. After the lapse of several days or even weeks, 
these scratching movements which result from pinching in the 
above-mentioned area, form the beginning of a complete epileptic 
attack. Hence the changes immediately produced by the division 
of a nerve are obviously not the direct cause of epilepsy, but they 
only form the beginning of a pathological process which is con- 



314 THE SIGNIFICANCE OF SEXUAL KEPRODUCTION 

ducted in a centripetal direction from the nerve to some centre 
which is apparently situated in the pons and medulla oblong-ata, 
although, according" to others ^, it is placed in the cortex of the 
cerebrum. Nothnagel ^ considers that certain changes, the nature 
of which is still entirely unknown, but which may be histological 
or perhaps solely molecular in character, must be produced, leading 
to an increased irritability of the grey matter of the centres con- 
cerned. 

Nothnagel thinks it possible or even probable that in those 
cases in which the division of nerves is followed by epilepsy, a 
neuritis ascendens — an inflammation passing along the nerves in a 
central direction — is the cause of the changes suggested by him 
in the epileptic centre. All our knowledge of bacteria and of the 
pathological processes induced by them, seems to indicate that such 
a neuritis ascendens, as is assumed by Nothnagel, would render 
important support to the hypothesis that the artificial epilepsy is 
due to infection. But when we further consider that the offspring 
of artificially epileptic animals may themselves become epileptic, 
although in most cases they suffer from a variety of other nervous 
diseases (in consequence of trophic paralysis), I hardly see how the 
facts can be rendered intelligible except by supposing that in these 
cases of what I may call traumatic epilepsy, we are dealing with 
an infectious disease caused by microbes which find their nutritive 
medium in the nervous tissues, and which bring about the trans- 
mission of the disease to the offspring by penetrating the ovum or 
the spermatozoon. 

Obersteiner found that the offspring were more frequently dis- 
eased when the mother was epileptic, rather than the father. This 
is readily intelligible when we remember that the ovnra. contains 
an immensely larger amount of substance than the spermatozoon, 
and can therefore be more frequently infected by microbes and can 
contain a greater number of them. 

Of course, I do not mean to assert that epilepsy always depends 
upon infection, or upon the presence of microbes in the nervous 
tissues. Westphal produced epilepsy in guinea-pigs by striking 

^ Compare Unvericht, ' E^erimentelle und klinische Untersuchungen uber die 
Epilepsie.' Berlin, 1883. With regard to the question of hereditary transmission, 
the part of the brain in which the epileptic centre is placed is of no importance. 

* Compare Ziemssen's ' Handbuch der spec. Pathologic und Therapie.' Bd. XII. 
2. Halfte; Artikel ' Epilepsie und Eklampsie.' Leipzig, 1877. 



IN THE THEORY OF NATURAL SELECTION. 315 

them once or twice sharply upon the head : the epileptic attack 
took place immediately and was afterwards repeated. It is obvious 
that the presence of microbes can have nothing- to do with such an 
attack, but the shock alone must have caused morphological and 
functional chang-es in the centres of the pons and medulla oblong-ata, 
identical with those produced by microbes in the other cases. 
Nothnagel also distinctly expresses the opinion that epilepsy ' does 
not depend upon one uniform and invariable histological change, 
but that the symptoms which constitute the disease may in all pro- 
bability be caused by various anatomical alterations, provided that 
they take place in parts of the pons and medulla which are mor- 
phologically and physiologically equivalent^.' Just as a sensory 
nerve produces the sensation of pain under various stimuli, such as 
pressure, inflammation, infection with the poison of malaria, etc., 
so various stimuli might cause the nervous centres concerned to 
develope the convulsive attack which, together with its after-effects, 
we call epilepsy. In Westphal's case, such a stimulus would be 
given by a powerful mechanical shock, in Brown- Sequard's experi- 
ments, by the penetration of microbes. 

However, quite apart from the question of the validity of this 
suggestion, we can form no conception as to the means by which 
an acquired morphological change in certain nerve-cells — a change 
which is not anatomical, and probably not even microscopical, but 
purely molecular in nature — can be possibly transferred to the 
germ-cells : for this ought to take place in such a manner as to 
produce in their minute molecular structure a change which, after 
fertilization and development into a new individual, would lead to 
the reproduction of the same epileptogenic molecular structure of 
the nervous elements in the grey centres of the pons and medulla 
oblongata as was acquired by the parent. How is it possible for all 
this to happen ? What substance could cause such a change in the 
resulting offspring after having been transferred to the egg or sperm- 
cell ? Perhaps Darwin's gemmules may be suggested ; but each 
gemmule represents a cell, while here we have to do with molecules 
or group's of molecules. We must therefore assume the existence 
of a special gemmule for each group of molecules, and thus the 
innumerable gemmules of Darwin's theory must be imagined as 
increased by many millions. But if we suppose that the theory 

^ 1. c, p. 269. 



316 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

of panjSfenesis is right, and that the gemmules really circulate in 
the body, accompanied by other g-emmules from the diseased parts 
of the brain, and that some of these latter pass into the germ- 
cells of the individual, — to what strange results would the further 
pursuit of this idea lead ? What an incomprehensible number of 
gemmules must meet in a single sperm- or germ -cell, if each of 
them is to contain a representative of every molecule or group 
of molecules which has formed part of the body at each period of 
ontogeny. And yet such is the unavoidable consequence of the 
supposition that acquired molecular states of certain groups of cells 
can be transmitted to the offspring. This supposition could only be 
rendered intelligible by some theory of preformation^, such as Dar- 
win's pangenesis ; for the latter theory certainly belongs to this 
category. We must assume that each single part of the body at 
each developmental stage is, from the first, represented in the germ- 
cell as distinct particles of matter, which will reproduce each part 
of the body at its appropriate stage as their turn for development 
arrives. 

I will only briefly indicate some of the inevitable contradictions 
in which we are involved by such a theory. One and the same 
part of the body must be represented in the germ- or sperm-cell 
by many groups of gemmules, each group corresponding to a 
different stage of development ; for if each part gives off gem- 
mules, which ultimately reproduce the part in the offspring, it is 
clear that special gemmules must be given off for each stage in 
the development of the part, in order to reproduce that identical 
stage. And Darwin quite logically accepts this conclusion in his 
provisional hypothesis of pangenesis. But the ontogeny of each 
part is in reality continuous, and is not composed of distinct and 

' It is generally known that the earlier physiologists believed in what was called 
the ' evolutionary theory,' or the ' theory of preformation.' This assumes that the 
germ contains, in a minute form, the whole of the fully-developed animal. All the 
parts of the adult are preformed in the germ, and development only consists in the 
growth of these parts and their more perfect arrangement. This theory was generally 
accepted until the middle of the last century, when Kaspar Friedrich Wolff brought 
forward the theory of ' epigenesis,' which since that time has been the dominant one. 
This assumes that no special parts of the germ are preformations of certain parts of 
the fully-developed animal, anvi that these latter arise by a series of changes in the 
germ, which gradually gives rise to them. In modem times the theory of preforma- 
tion has been revived in a less crude form, as is shown by the ideas of Nageli, and 
by Darwin's 'pangenesis.' — A. W., 1888. 



IN THE THEORY OF NATURAL SELECTION. 317 

separate stages. We imagine these stages as existing in the con- 
tinuous course of ontogeny; for here, as in all departments of 
nature, we make artificial divisions in order to render possible a 
general conception, and to gain fixed points in the continuous 
changes of form which have in reality occurred. Just as we dis- 
tinguish a sequence of species in the course of phylogeny, although 
only a gradual transition, not traversed by sharp lines of demar- 
cation, has taken place, so also we speak of the stages of ontogeny, 
although we can never point out where any stage ends and another 
begins. To imagine that each single stage of a part is present 
in the germ, as a distinct group of gemmules, seems to me to be a 
childish idea, comparable to the belief that the skull of the young 
St. Laurence exists at Madrid, while the adult skull is to be found 
in Rome. 

We are necessarily driven to such conceptions if we assume that 
the transmission of acquired characters takes place. A theory of 
preformation alone affords the possibility of an explanation : an 
epigenetic theory is utterly unable to render any assistance in 
I'eaching an interpretation. According to the latter theory, the 
germ does not contain any preformed gemmules, but it possesses, 
as a whole, such a chemical and molecular constitution that 
under certain circumstances, a second stage is produced from 
it. For example, the two first segmentation spheres may be re- 
garded as such a second stage ; these again possess such a con- 
stitution that a certain third stage, and no other, can arise from 
them, forming the four first segmentation spheres. At each of 
these stages the spheres produced are peculiar to a distinct species 
and a distinct individual. From the third stage a fourth arises, 
and so on, until the embryo is developed, and still later the mature 
animal which can reproduce itself. No one of the parts of such 
an animal was originally present as distinct parts in the egg 
from which it was developed, however minute we may imagine 
these parts to be. If now an inherited peculiarity shows itself in 
any organ of the mature animal, this will be the consequence of 
the preceding developmental stages, and if we were able to inves- 
tigate the molecular structure of all these stages as far back as 
' the egg-cell, we should trace back to the latter some minute 
difference of molecular constitution which would distinguish it 
from any other egg-cell of the same species, and was destined 



318 THE SIGNIFICANCE OP SEXUAL REPRODUCTION 

to be the cause of the subsequent appearance of the peculiarity 
in the mature animal. It is only by the aid of some such hypo- 
thesis that we can conceive the cause of hereditary individual dif- 
ferences and the tendencies towards hereditary diseases. Hereditary 
epilepsy would be intelligible in this way, that is, when the disease 
is cong-enital and not due to the presence of microbes, as is pre- 
sumably the case with aiiificially induced epilepsy. 

The question now arises as to whether we can conceive the 
communication of such traumatic and therefore acquired epilepsy 
to the g-erm-cells. This is obviously impossible under the epi- 
genetic theory of development described above. In what way 
can the germ-cells be affected by molecular or histological changes 
in the pons varolii and medulla oblongata ? Even if we assume, 
for the sake of argument, that the central nervous system exercises 
trophic influences upon the germ-cells, and that such influences 
may consist of something more than variations in nutritive con- 
ditions, and may even include the power of altering the molecular 
constitution of the germ-plasm in spite of its usual stability; even 
if we concede these suppositions, how is it conceivable that the 
changes produced would be of the exact nature and in the exact 
direction necessary in order to confer upon the germ-plasm the 
molecular sti'ucture of the first ontogenetic stage of an epileptic 
individual ? How can the last ontogenetic stage of the ganglion 
cells in the pons and medulla of such an individual, stamp upon 
the germ-plasm in the germ-cells of the same animal — ^not indeed 
the peculiar structure of the stage itself — but such a molecular 
constitution as will ensure the ultimate appearance of epilepsy 
in the offspring? The theory of epigenesis does not admit that 
the parts of the full-grown individual are contained in the germ as 
preformed material particles, and therefore this theory cannot allow 
that anything is added to the germ-plasm ; but in accepting the 
above-made supposition, Ave are compelled to assume that the mole- 
cular structure of the whole of the germ-plasm is changed to a 
slight extent. 

Nageli is quite right in maintaining that the solid protoplasm 
alone, as opposed to the fluid part, i. e. that part of the protoplasm 
which has passed into solution, can act as the bearer of hereditary 
tendencies. This appears to be undoubtedly proved by the fact that 
the amount of material provided by the male parent for the de- 



IN THE THEORY OF NATURAL SELECTION. 319 

velopment of an embryo is in almost all animals far smaller than 
the amount provided by the female parent. 

In Mammalia the share contributed by the father probably only 
forms about one hundred-billionth part of that contributed by the 
mother, and yet nevertheless the influence of the former in he- 
redity is on an average equal to that exerted by the latter^. Now, 
from the point of view of epigenesis, no molecule of the brain of 
an epileptic animal can reach the germ-cell except in a state of 
solution, and therefore no direct increase in the germ-plasm can 
be referred to such molecules, quite apart from the fact that such 
addition, even if possible, could not be of any value, because the 
last stage of the epileptic tendency must be represented in the 
nerve-cells and nerve-fibres of the diseased brain, while the first 
stage ought to be represented in the germ-cell. 

It may be safely asserted that according to the theory of epigenesis 
the germ-cells cannot be influenced except as regards their nutri- 
tion. Nutritive changes may be imagined to occur through the 
varying trophic influence of the nervous system upon the sexual 
organs, but the structure of the germ -plasm cannot be altered by 
mere nutritive changes, or at all events it cannot be altered in 
that distinct and definite direction which is required by the sup- 
posed transmission of acquired epilepsy. 

Thus the transmission of artificially produced epilepsy can neither 
be explained upon the epigenetic theory, nor upon the theory of 
preformation ; it can only be rendered intelligible if we suppose 
that the appearance of the disease in the oflspring depends upon 
the introduction and presence of living germs, viz. of microbes. 
The supposed transmission of this artificially produced disease is 
the only definite instance which has been hitherto brought forward 
in support of the transmission of acquired characters. I believe 
that I have shown that such support is deceptive, not because there 
is any uncertainty about the fact of the transmission itself, but 
because it is a transmission which cannot depend upon heredity, 
and is in all probability due to infection. 

Ever since I began to doubt the transmission of acquired cha- 
racters, I have been unable to meet with a single instance which 
could shake my conviction. There were many instances in which 
hereditary transmission was clearly established, but in none of them 

^ Nageli, 1. c. p. no. 



320 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

was there any reason to sappose that the characters transmitted were 
really acquired. For example, Fritz Miiller has recently informed 
me of an instance in which he believes that there can be no doubt 
of the transmission of acquired characters. His observations are 
so interesting in several respects that I will quote them here. He 
says in his letter, ' Among the bastards of two species of Abutilon, 
in which I had never observed hexamerous flowers, there was 
a single plant with a few such blossoms. As these flowers are 
sterile with the pollen of the same plant, I was obliged to fer- 
tilize it with pollen from another plant bearing only pentamerous 
flowers, in order to obtain seeds from the former. For three weeks 
I examined all the flowers from a plant grown from such seed, 
finding 145 pentamerous, 103 hexamerous, and 13 heptamerous 
flowers. I examined similarly the flowers of another plant pro- 
duced from seed obtained from pentamerous flowers from the same 
parent plants. There were 454 pentamerous and 6 hexamerous 
flowers, and hence only 1*3 per cent, of the latter kind.' 

It must certainly be admitted that the large proportion of ab- 
normal hexamerous flowers depends upon heredity in the instance 
first quoted ; but the hexamerous condition is not an acquired 
character ; it is merely the first appearance of a new innate^ 
character. It is not due to the reaction of the vegetable organism 
under some external stimulus, for it appeared in a plant exposed to 
conditions similar to those which acted upon the other plant which 
only produced the normal pentamerous flowers. It must therefore 
have resulted from the tendencies which were present in the germ 
from which the plant itself developed, either as a spontaneous 
change in the germ-plasm or through the combination of two 
parental germ-plasms — a combination which may lead to the 
appearance or the reality of a new character. We know that the 
germ-plasm of each individual is not a simple substance, but pos- 
sesses a very complex composition, for it consists of a number of 
ancestral germ-plasms represented in very different proportions. 
•Now, although we cannot learn anything directly about the pro- 
cesses of growth of the germ-plasm, and its resulting ontogenetic 
stages, yet we do know, chiefly from observations upon man, that 
the characters of ancestors appear in the offspring in very different 
combinations and in very different degrees of strength. This may, 
perhaps, be explained by assuming that in the union of parental 



IN THE THEORY OP NATURAL SELECTION. 321 

germ-plasms which takes place at fertilization, the contained an- 
cestral g-erm-plasms unite in different ways, and thus come to grow 
with different strengths. Certain ancestral germ-plasms will meet 
and together produce a double effect : other opposed germ-plasms 
will neutralize each other ; and between these two extremes all in- 
termediate conditions will occur. And these combinations will not 
only take place at fertilization, but also at every stage of the whole 
ontogenetic history, for each stage is represented by its idioplasm, 
which is itself composed of ancestral idioplasms. 

We do not yet know enough to be able to prove in detail 
the manner in which new characters may arise from such a com- 
bination of different kinds of germ-plasm. And yet it appears to 
me that such a view, e. g. in the case of the variation of buds, is by 
far the most natural. There is indeed a single example in which 
we can, to some extent, understand how it is that a new character 
may arise by these means. Certain canary-birds have a tuft of 
feathers on the head, but if two such birds are paired, their 
descendants are generally bare-headed, instead of having larger 
tufts ^. The formation of a tuft depends upon the fact that the 
feathers are scanty and in fact absent from part of the skin of the 
head. Now when the scanty plumage of both parents is combined 
in the offspring the latter is bare-headed. Hence by the com- 
bination of ancestral characters a new character (bare-headedness) is 
produced, and one which is hardly likely to have ever occurred in 
the ancestors of existing canaries. 

We do not know the causes which have been in operation when 
a flower possesses one petal more than the usual number, any more 
than we can explain why it is that one star-fish has five and 
another six rays. We cannot unravel the details of the mysterious 
relationship between two parent germ-plasms, each of which is 
composed of a countless number of ancestral germ-plasms from the 
first and second back to the nth. degree. But we can neverthe- 
less maintain in a general way that such irregularities are the 
result of this complex struggle between the germ-plasms in the 
ovum and the idioplasms in the subsequent stages of the de- 
veloping organism, and that they are not the result of external 
influences. 

' See Darwin, 'The Variation of Animals and Plants under Domestication.' 1875. 
Vol. I. p. 311. 

Y • 



322 THE SIGNIPICANCE OP SEXUAL REPRODUCTION 

If, however, acquired characters are brought forward in con- 
nexion with the question of the transformation of species, the term 
' acquired ' must only be applied to those characters which do not 
arise from within the organism, but which arise as the reaction of 
the organism under some external stimulus, most commonly as the 
consequence of the increased or diminished use of an organ or part. 
We have then to learn whether the altered conditions of life, by 
forcing an organism to adopt new habits, can by such means lead 
directly, and not indirectly through natural selection, to the 
transformation of the species ; or whether the effects of increased 
or diminished use of certain parts, implied by the new habits, are 
restricted to the individual itself, and therefore powerless to effect 
any direct modification of the species. 

Fritz Miiller's observation is also interesting in another re- 
spect : it appears to controvert ihy views upon heredity as expressed 
in the theory of the continuity of the germ-plasm. If a single 
flower can transmit to its descendants special peculiarities which 
were not possessed by its ancestors, we seem to be driven to the 
conclusion that the ancestral germ-plasm has not passed into the 
flower in question, but that new germ-plasm has been formed, 
inasmuch as the new characters are derived from the flower itself, 
and not from any of its ancestors. I think, however, that the 
observation admits of another interpretation : a specimen of Abu- 
tilon with many hundred flowers is not a single individual, but a 
colony consisting of numerous individuals which have arisen by 
budding from the first individual developed from the seed. 

I have not hitherto considered budding in relation to my 
theories, but it is obvious that it is to be explained from my point 
of view, by supposing that the germ-plasm which passes on into a 
budding individual consists not only of the unchanged idioplasm of 
the first ontogenetic stage (germ-plasm), but of this substance altered, 
so far as to correspond with the altered structure of the individual 
which arises from it — viz. the rootless shoot which springs from the 
stem or branches. The alteration must be veiy slight, and perhaps 
quite insignificant, for it is possible that the differences between the 
secondary shoots and the primary plant may chiefly depend upon 
the changed conditions of development, which takes place beneath 
the earth in the latter case, and in the tissues of the plant in 
the former. Thus we may imagine that the idioplasm, when it 



IN THE THEORY OF NATURAL SELECTION. 323 

developes into a flowering shoot, produces at the same time the 
germ-cells which are found in the latter. We thus approach an 
understanding of Fritz Muller's observation ; for if the whole shoot 
which produces the flower arises from the same idioplasm which 
also forms its germ-cells, we can readily understand why the latter 
should contain the same hereditary tendencies which were previously 
expressed in the flower which produced them. The fact that varia- 
tions may occur in a single shoot depends upon the changes 
explained above, which occur in the idioplasm during the course 
of its growth, as a result of the varying proportions in which the 
ancestral idioplasms may be contained in it. 

Fritz Miiller's observation affords a beautiful confirmation of 
this view, for if the flower itself transmitted the hexamerous 
condition to its germ-cells, we could not understand why some of 
the extremely rare hexamerous flowers were produced by the cross- 
ing of two pentamerous flowers, in the control experiment. An 
explanation of this fact can only be found in the assumption that 
the germ-plasm contained in the mother plant, during its growth 
and consequent distribution through all the branches of the colony, 
became arranged into a combination of idioplasms, which, whenever 
it predominated (as it did at certain places), necessarily led to the 
formation of hexamerous flowers. I will not consider here the 
question as to whether this combination is to be looked upon as an 
instance of reversion, or whether it represents something new. Such 
a question is of no importance for our present pm'pose ; but the 
hexamerous flowers of the control experiment prove, in my opinion, 
that germ -plasm containing the requisite combination was dis- 
tributed in the mother plant and also existed, but in insufficient 
amount, in shoots which did not produce any hexamerous flowers. 



Appendix V. On the Origin of PArvTHENOGENESis ^ 

The transformation of heterogeny into pure parthenogenesis has 
obviously been produced by other causes as well as by those mentioned 
in the main part of this paper. Other and quite different circum- 
stances have also had a share in its production. Pure parthenogenesis 
may be produced without the intermediate condition of heterogeny. 

* Appendix to page 290. 

Y a 



324 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

Thus, for example, the pure and exclusive parthenogenesis with 
which the large Phyllopod crustacean, Apta^ is reproduced at most, 
of its habitats, has not arisen from the loss of previously existent 
sexual generations, but simply from the non-appearance of males, 
accompanied by the simultaneous acquisition of the power, on the 
part of the females, of producing eggs which do not require 
fertilization. This is proved by the fact that males occur in certain 
scattered colonies of this species, and sometimes they are even 
present in considerable numbers. But even if we were not aware 
of these facts, the same conclusions might nevertheless have been 
drawn from the fact that Apus produces eggs of only one form 
— viz. resting eggs with hard shells. In every case in which par- 
thenogenesis has been first introduced in alternation with sexual 
reproduction, the resting eggs are produced by the latter genera- 
tions, while the parthogenetic generations produce eggs with thin 
shells, in which the embryo developes and hatches very rapidly. 
In this way parthenogenesis leads to a rapid increase of the colony. 
In Ajpus such increase in the number of individuals is gained in an 
entirely different manner, viz. by the fact that all the animals 
become females, which produce eggs at a very early age, and con- 
tinue producing them in increasing fertility for the whole of their 
life. In this manner an enormous number of eggs collects at the 
bottom of the pool inhabited by the colony, so that after it has 
dried up, in spite of loss from various destructive agencies, therq 
will still remain a sufficiency of eggs to reproduce a numerous 
colony, as soon as the pool has filled again. 

This form of parthenogenetic reproduction is especially well 
suited to the needs of species inhabiting small pools which entirely 
depend upon rain-fall, and which may disappear at any time. In 
these cases the time during which the colony can live is often too 
short to permit the production of several generations even from 
rapidly developing summer- eggs. Under these circumstances the 
pool would often suddenly dry up before the series of parthenogenetic 
generations had been run thi'ough, and hence before the appear- 
ance of the sexual generation and resting eggs. In all such cases 
the colony would be exterminated. 

This consideration might lead us to think that Crustacea, such 
as the I)aj)hnidae, which develope by means of het^rogeny, would 
hardly be able to exist in small pools filled by the rain ; but here 



IN THE THEORY OF NATURAL SELECTION. 325 

also nature has met the difReulty by another adaptation. As I 
have shown in a previous paper ^j the heterogeny of the species of 
Baphnidae which inhabit such pools is modified in such a manner, 
that only the first generation produced from the resting" eggs 
consists of purely parthenogenetic females, while the second includes 
many sexual animals, so that resting eggs are produced and laid, 
and the continuance of the colony is secured a few days after it has 
been first founded ; viz. after the appearance of the first generation. . 

But it is also certain that in the Baphnidae, heterogeny may 
pass into pure parthenogenesis by the non-appearance of the sexual 
generations. This seems to have taken place in certain species of 
Bosmina and Chydorus, although perhaps only in those colonies of 
which the continuance is secured for the whole year ; viz. those 
which inhabit lakes, water-pipes, or wells in which the water 
cannot freeze. In certain insects also (e. g. JRhodites rosae) pure 
parthenogenesis seems to be produced in a similar manner, by the 
non-appearance of males. 

But the utility which we may look upon as the cause of partheno- 
genesis is by no means so clear in all cases. Sometimes, especially 
in certain species of Ostracoda, its appearance seems almost like a 
mere caprice of nature. In this group of the Crustacea, one species 
may be purely parthenogenetic, while a second reproduces itself 
by the sexual method, and a third by an alternation of the two 
methods : and yet all these species may be very closely allied and 
may frequently live in the same locality and apparently with the 
same habit of life. But it must not be forgotten that it is only 
with the greatest difficulty that we can acquire knowledge about 
the details of the life of these minute forms, and that where we can 
only recognize the appearance of identical conditions, there may be 
highly important differences in nutrition, habits, enemies and the 
means by which they are resisted, and in the mode by which 
the prey is captured — circumstances which may place two species 
living in the same locality upon an entirely different basis of 
existence. It is not merely probable that this is the case ; for the 
fact that certain species have modified their modes of reproduction 
is in itself a sufficient proof of the validity of the conclusions which 
have just been advanced. 

* Weismann, * Naturgeschichte der Daphnoiden,' Zeitschrift f. wiss. Zool. XXIII. 
1879. 



326 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

The fact that different methods of reproduction may obtain in 
different colonies of the same species, although with thoroughly 
identical habits, may depend upon differences in the external con- 
ditions (as in Bosmina and Chydorus mentioned above), or upon the 
fact that the transition from sexual to parthenogenetic reproduction 
is not effected with the same ease and rapidity in all the colonies 
of the same species. As long as males continue to make their 
appearance in a colony of Apus, sexual reproduction cannot wholly 
disappear. Although we are unable to appreciate, with any degree 
of certainty, the causes by which sex is determined, we may never- 
theless confidently maintain that such determining influences may 
be different in two widely separated colonies. As soon, however, as 
parthenogenesis becomes advantageous to the species, securing its 
existence more eflaciently than sexual reproduction, it will not only 
be the case that the colonies which produce the fewest males will gain 
advantage, but within the limits of the colony itself, those females 
will gain an advantage which produce eggs that can develope without 
fertilization. When the males are only present in small numbers, it 
must be very uncertain whether any given female will be fertilized : 
if therefore the eggs of such a female required fertilization in order 
to develope, it is clear that there would be great danger of entire failure 
in this necessary condition. In other words : — as soon as any females 
begin to produce eggs which are capable of development without 
fertilization, from that very time a tendency towards the loss of 
sexual reproduction springs into existence. It seems, however, that 
the power of producing eggs which can develope without fertiliza- 
tion is very widely distributed among the Arthropoda. 



Appendix VI. W. K. Brooks' Theory of Heredity*. 

The only theory of heredity which, at any rate in one point, 
agrees with my own, was brought forward two years ago by W. K. 
Brooks of Baltimore ^. The point of agreement lies in the fact that 
Brooks also looks upon sexual reproduction as the means employed 
by nature in order to produce variation. The manner in which he 
supposes that the variability arises is, however, very different from 

' Appendix to page 277. 

' Compare W. K. Brooks, ' The Law of Heredity, a Study of the Cause of Variation, 
and the Origin of living Organisms.' Baltimore, 1883. 



IN THE THEORY OP NATURAL SELECTION. 327 

that sug-g-ested in my theory, and our fundamental conceptions are 
also widely diverg-ent. "While I look upon the continuity of the 
g-erm-plasm as the foundation of my theory of heredity, and there-^ 
fore believe that permanent hereditary variability can only have 
arisen throug'h some direct chang'e in the g-erm-plasm effected by 
external influences, or following" from the varied combinations which 
are due to the mixture of two individually distinct g-erm-plasms 
at each act of fertilization, Brooks, on the other hand, bases his 
theory upon the transmission of acquired characters, and upon the 
idea which I have previously called 'the cyclical development of 
the g-erm-plasm.' 

Brooks' theory of heredity is a modification of Darwin's pan- 
genesis, for Brooks also assumes that minute g-emmules are thrown 
off by each cell in the body of the higher organisms ; but such 
gemmules are not emitted always, and under all circumstances, 
but only when the cell is subjected to unaccustomed conditions. 
During the persistence of the ordinary conditions to which it is 
adapted, the cell continues to perform its special functions as part 
of the body, but as soon as the conditions of life become unfavour- 
able and its functions are disturbed, the cell ' throws off minute 
particles which are its germs or gemmules.' 

These gemmules may then pass into any part of the organism ; 
they may penetrate the ova in the ovary, or may enter into a bud, 
but the male germ-cells possess a special power of attracting them 
and of storing them up within themselves. 

According to Brooks, variability arises as a consequence of the 
fact that each gemmule of the sperm-cell unites, during fertiliza- 
tion, with that part of the ovum which, in the course of develop- 
ment, is destined to become a cell corresponding to that from 
which the gemmule has been derived. 

Now, when this cell developes in the offspring, it miist, as a 
hybrid, have a tendency to vary. The ova themselves, as cells, 
are subject to the same laws ; and the cells of the organism will 
continue to vary until one of the variations is made use of by 
natural selection. As soon as this is the case, the organism 
becomes, ipso facto, adapted to its conditions ; and the production 
of gemmules ceases, and with it the manifestation of variability 
itself, for the cells of the organism then derive the whole of their 
qualities from the egg, and being no longer hybrid, have no 



328 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

tendency to vary. For the same reason the ova themselves will 
also cease to vary, and the favourable variation will be transmitted 
from generation to generation in a stereotyped succession, until 
unfavourable conditions arise, and again lead to a fresh disposition 
to vary. 

In this way Brooks ^ attempts to mediate between Darwin and 
Lamarck, for he assumes, on the one hand, that external influences 
render the body or one of its parts variable, while, on the other 
hand, the nature of the successful variations is determined by 
natural selection. There is, however, a difference between the 
views of Brooks and Darwin, although not a fundamental difference. 
Darwin also holds that the organism becomes variable by the opera- 
tion of external influences, and he farther assumes that changes 
acquired in this way can be communicated to the germ and trans- 
mitted to the offspring. But according to his hypothesis, every 
part of the organism is continually throwing off gemmules which 
may be collected in the germ-cells of the animal, while, according 
to Brooks, this only takes place in those parts which are placed 
under unfavourable conditions or the function of which is in some 
way disturbed. In this manner the ingenious author attempts to 
diminish the incredible number of gemmules which, according to 
Darwin's theory, must collect in the germ-cells. At the same time 
he endeavours to show that those parts must always vary which 
are no longer well adapted to the conditions of life. 

I am afraid, however, that Brooks is confounding two things 
which are in reality very different, and which ought necessarily 
to be treated separately if we wish to arrive at correct conclusions : 
viz., the adaptation of a part of the body to the body itself, and 
its adaptation to external conditions. The first of these adapta- 
tions may exist without the second. How can those parts become 
variable which are badly adapted to the external conditions, but 
are nevertheless in complete harmony with the other parts of the 
body? If the conditions of life, of the cells which constitute the 
part in question must become unfavourable, in order that the 
gemmules which produce variation may be thrown off, it is obvious 
that such a result would not occur in the case mentioned above. 
Suppose, for example, that the spines of a hedgehog are not suflS- 
ciently long or sharply pointed to afford protection to the animal, 

1 1. c, p. 82. 



IN THE THEORY OF NATURAL SELECTION. 329 

how could sucli an unfavourable development afford the occasion 
for the throwing- off of gemmules, and a resulting variability 
of the spines, inasmuch as the epidermic tissue in which these 
structures arise, remains under completely normal and favourable 
conditions, whatever length or sharpness the spines may attain? 
The conditions of the epidermis are not unfavourably affected 
because, as the result of short and blunt spines, the number of 
hedgehogs is reduced to far below the average. Or consider the 
case of a brown caterpillar which would gain great advantage by 
becoming green ; what reason is there for believing that the cells 
of the skin are placed in unfavourable conditions, because, in 
consequence of the brown colour, far more caterpillars are detected 
by their enemies, than would have been the case if the colour 
were green ? And the case is the same with all adaptations. 
Harmony between the parts of the organism is an essential con- 
dition for the existence of the individual. If it is wanting, the 
individual is doomed ; but such harmony between any one part and 
all others, i. e. proper nutrition for each part, and adequate per- 
formance of its proper function, can never be disturbed by the 
fact that the part in question is insufficiently adapted to the outer 
conditions of life. According to Darwin, all the cells of the body 
are continually throwing off gemmules, and against such an 
assumption no similar objection can be raised. It can only be 
objected that the assumption has never been proved, and that it 
is extremely improbable. 

A further essential difference between Darwin's theory of 
pangenesis and Brooks' hypothesis lies in the fact that Brooks 
holds that the male and female germ-cells play a different part, 
and that they tend to become charged with gemmules in different 
degrees, the egg-ca'l containing a far smaller number than the 
sperm-cell. According to Brooks the egg-cell is the conservative 
principle which brings about the permanent transmission of the 
true characters of the race or species, while he believes that the 
sperm-cell is the progressive principle which causes variation. 

The transformation of species is therefore believed to take place, 
for the most part, as follows: — those parts which are placed in 
unfavourable conditions by the operation of external influences, 
and which have varied, throw off gemmules which reach the 
sperm-cells, and the latter by fertilization further propagate the 



330 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 

variation. An increase of variation is produced because the gem- 
mules which reach the egg through the sperm-cell may unite or con- 
jugate with parts of the former which are not the exact equivalents 
of the cells from which the gem mules arose, but only very nearly 
related to them. Brooks calls this 'hybridization,' and he con- 
cludes that, just as hybrids are more variable than pure species, so 
such hybridized cells are also more variable than other cells. 

The author has attempted to work out the details of his 
theory with great ingenuity, and as far as possible to support his 
assumptions by facts. Moreover, it cannot be denied that there 
are certain facts which seem to indicate that the male germ-cell 
plays a different part from that taken by the female germ-cell in 
the formation of a new organism. 

For example, it is well known that the offspring of a horse and 
an ass is different when the male parent is a horse from what it is 
when the male parent is an ass. A stallion and a female ass 
produce a hinny which is more like a horse, while a male ass and 
a mare produce a mule which is said to be more like an ass^. I 
will refrain from considering here the opinion of several authors 
(Darwin, Flourens, and Bechstein) that the influence of the ass is 
stronger in both cases, only predominating to a less extent when 
the ass is the female parent ; and I will for the sake of brevity 
accept Brooks' opinion that in these cases the influence of the 
father is greater than that of the mother. Were this so in all 
cross-breeding between different species and in all cases of normal 
fertilization, we should be compelled to conclude that the influ- 
ences of the male and female germ -plasms upon the offspring 

^ This seems to be the general opinion (see the quotation from Huxley in Brooks' 
' Heredity,' p. 127) ; but I rather doubt whether there is such a constant difference 
between mules and hinnies. Furthermore, I cannot accept the opinion that mules 
always resemble the ass more than the horse. I have seen many mules which bore 
a much stronger likeness to the latter. I believe that it is at present impossible to 
decide whether there is a constant difference between mules and hinnies, because the 
latter are very rarely seen, and because mules are extremely variable. I attempted 
to decide the question last winter by a careful study of the Italian mules, but I 
could not come across a single hinny. These hybrids are very rarely produced, 
because it is believed that they are extremely obstinate and bad-tempered. I after- 
wards saw two true hinnies at Professor Kuhn's Agricultural Institute at Halle. 
These hinnies by no means answered to the popular opinion, for they were quite 
tractable and gcod-tempered. They looked rather more like horses than asses, 
although they resembled the latter in size. In this case it was quite certain that 
one parent was a stallion and the other a female ass. — A. W. 18S9. 



IN THE THEOKY OF NATURAL SELECTION. 331 

differ at any rate in strength. But this is by no means always 
the case, for even in horses the reverse may occur. Thus it is 
stated that certain female race-horses have always transmitted 
their own peculiarities, while others allowed those of the stallion 
to preponderate. 

In the himian species the influence of the mother preponderates 
quite as often as that of the father, although in many families most 
of the children may take after either parent. There is nevertheless 
hardly any large family in which all the children take after the 
same parent. If we now try to explain the preponderating in- 
fluence of one parent by the supposition of a greater strength in 
hereditary power, without first inquiring after some deeper cause, 
I think the only conclusion warranted by the facts before us is 
that this power is rarely or never equal in both of the conjugating 
germ-cells, but that even within the same species, sometimes the 
male and sometimes the female is the stronger, and that the strength 
may even vary in the different offspring of the same individuals, 
as we so frequently see in human families. The egg-cells of the 
same mother which ripen one after the other, and also the sperm- 
cells of the same father, must therefore present variations in the 
strength of their hereditary power. It is then hardly to be wondered 
at that the relative hereditary power of the germ-cells in different 
species should vary, although we cannot as yet understand why 
this should be the case. 

It would not be very difficult to render these facts intelligible 
in a general way by an appeal to physiological principles. The 
quantity of germ-plasm contained in a germ-cell is very minute, 
and together with the idioplasms of the various ontogenetic 
stages to which it gives rise, it must be continually increased by 
assimilation during the development of the organism. If now this 
power of assimilation varied in intensity, a relatively rapid growth 
of the idioplasm derived from one of the parents would ensue, 
and with it the preponderance of the hereditary tendencies of 
the parent in question. Now, it is obvious that no two cells of 
the same kind are entirely identical, and hence there must be 
differences in their powers of assimilation. Thus the varying 
hereditary powers of the egg-cells produced from the same ovary 
become explicable, and still more easily the varying powers of the 
germ-cells produced in the ovaries or testes of different individuals 



332 THE SIGNIFICANCE OP SEXUAL REPRODUCTION, ETC. 

of the same species ; most easily of all the differences observable 
in this respect between the germ-cells of different species. 

Of course, this hereditary power is always relative, as may be 
easily proved by cross-breeding* between diflPerent species and races. 
Thus when a fantail pigeon is crossed with a laugher, the characters 
of the former preponderate, but when crossed with a pouter the 
characters of the latter preponderate \ The facts afforded by cross- 
breeding between hybrids and one of the pure parent species, 
together with a consideration of the resulting degree of variability, 
seem to me to be even more unfavourable to Brooks' view. They 
appear to me to admit of an interpretation different from that 
brought forward by him ; and when he proceeds to make use of 
secondary sexual characters for the purpose of his theory, I believe 
that his interpretation of the facts can be easily controverted. It 
is hardly possible to conclude that variability is due to the male 
parent, because the males in many species of animals are more 
variable, or deviate further from the original type, than the females. 
It is certainly true that in many species the male sex has taken 
the lead in processes of transformation, while the female sex has 
followed, but there is no difficulty in finding a better explanation 
of the fact than that afforded by the assumption ' that something 
within the animal compels the male to lead and the female to follow 
in the evolution of new breeds.' Brooks has with great ingenuity 
brought forward certain instances which cannot be explained with 
perfect confidence by Darwin's theory of sexual selection, but this 
hardly justifies us in considering the theory to be generally in- 
sufficient, and in having recourse to a theory of heredity which is 
as complicated as it is improbable. The whole idea of the passage 
of gemmules from the modified parts of the body into the germ- 
cells is based upon the unproved assumption that acquired characters 
can be transmitted. The idea that the male germ-cell plays a 
different part from that of the female, in the construction of the 
embryo, seems to me to be untenable, especially because it conflicts 
with the simple observation that upon the whole human children 
inherit quite as much from the father as from the mother. 

* Darwin, ' Variation of Animals and Plants under Domestication/ 1875, ^o^- II- 
p. 41. 



VI. 

ON THE NUMBER OF POLAE BODIES AND 
THEIE SIGNIFICANCE IN HEEEDITY. 

1887. 



ON THE NUMBER OF POLAR BODIES AND 
THEIR SIGNIFICANCE IN HEREDITY. 



PREFACE. 

The following- paper stands in close relation to a series of short 
essays which I have published from time to time since the year 
1 88 1. The first of these treated of ' The Duration of Life,' and the 
last of ' The Significance of Sexual Reproduction.' The present 
essay is most intimately connected with that upon ' The Continuity 
of the Germ-plasm/ and has, in fact, grown out of the explanation 
of the meaning of polar bodies in the animal egg, brought forward 
in that essay. The explanation rested upon a trustworthy and solid 
foundation, as I am now able to maintain with even greater con- 
fidence than at that time. It rested upon the idea that in the egg- 
cell, a cell with a high degree of histological diSerentiation, two 
difierent kinds of nuclear substance exert their influence, one after 
the other. But continued investigation has shown me that the 
explanation built upon this idea is only correct in part, and that it 
does not exhaust the full meaning of the formation of polar bodies. 
In the present essay I hope to complete the explanation by the 
addition of essential elements, and I trust that, at the same time, 
I shall succeed in throwing new light upon the mysterious problems 
of sexual reproduction and parthenogenesis. 

It is obvious that this essay can only contain an attempt at an 
explanation, an hypothesis, and not a solution which is above 
criticism, like the results of mathematical calculation. But no 
biological theory of the present day can escape a similar fate, for 
the mathematical key which opens the door leading to the secrets of 
life has not yet been found, and a considerable period of time must 
elapse before its discovery. But although I can only ofier an hypo- 



336 ON THE NUMBER OF POLAR BODIES ETC. 

thesis, I hope to be able to show that it has not been rashly adopted, 
but that it has grown in a natural manner from the secure founda- 
tion of ascertained facts. 

Nothing impresses the stamp of truth upon an hypothesis more 
than the fact that its light renders intelligible not only those facts 
for the explanation of which it has been framed, but also other 
and more distantly related groups of phenomena. This seems to 
me to be the case with my hypothesis, since the interpretation of 
polar bodies and the ideas derived from it unite from very different 
points of view, the facts of reproduction, heredity and even the 
transformation of species, into a comprehensive system, which 
although by no means complete, is nevertheless harmonious, and 
therefore satisfactory. 

Only the most essential elements of the new facts which form 

the foundation of the views developed in this essay will be briefly 

mentioned. My object is to show all the theoretical bearings of 

these new facts, not to describe them in technical detail. Such 

a description accompanied by the necessary figures will shortly be 

ffiven in another place ^. 

A. W. ■ 

Freiburg i. Br., May 30, 1887. 

' See Berichten der Naturforschenclen Gesellschaft zn Freibm^ i. B., Band III. 
(1887) Heft I, 'Ueber die Bildung der Kichtungskcirper bei thierischen Eiern,' by 
August Weismaun and C. Ischikawa. 



ON THE NUMBER OF POLAR BODIES, &c. 



CONTENTS. 

PAGK 

I. Parthenogenetic and Sexual Egg 339 

The process of the formation of polar bodies very widely distributed . 339 

The significance of polar bodies according to Minot, Balfour, and van 

Beneden . . . . H . • . . . • 340 

— — My hypothesis of the removal of the histogenetic part of the nucleus . 341 

- — ^Confirmation by the discovery of polar bodies in parthenogenetic eggs . 345 
Parthenogenetic eggs form only ojic polar body, while eggs requiring 

fertilization form two . . . . . . . . ' . 346 

Parthenogenesis depends upon the fact that the part of the nucleus 
which is expelled from sexual eggs in the second polar body, remains 

in the egg ........... 348 

History of this discovery 349 

II. Significance of the second Polar Body 352 

Eefutation of Minot 's theory . . . . . . . -353 

The second division of the nuclear spindle involves a reduction of the 

ancestral germ-plasms . . . . . . . . •355 

The theoretical necessity for such reduction ...... 356 

Phyletic origin of the germ- plasms in existing species . . . -357 
The necessary reduction takes place by a special form of nuclear division 35S 
The division which causes this reduction has probably been already 

observed ........... 360 

'Van Beneden's and Carnoy's observations ...... 360 

Two different physiological effects of karyokinesis ..... 364 

Significance of direct nuclear division ....... 365 

Arguments in support of the view that the division of the egg-nucleus 
which causes reduction must occur at the end of ovogenetic develop- 
ment 367 

Such nuclear division is to be found in the formation of the second polar 

body 368 

History of the origin of this view . 368 

III. The Foregoing Considerations applied to the Male Germ-cells . 370 

The male germ-cells also require division in order to reduce the ancestral 

germ-plasms . . . . . . . . . . -37° 

The germ-plasms of the parents must be contained in the germ-plasm 

of the offspring .......... 370 

Advantages which the egg gains by the late occurrence of the ' reducing 

division' . . . . . . . . . . -37^ 

The causes of unequal division in the formation of polar bodies . -373 
These causes do not apply to the sperm-cell ...... 373 

Different kinds of nuclear division occur in spermatogenesis . . . 375 
Some of these may be interpreted as ' reducing divisions ' . . . 375 
The paranucleus ('Nebenkern') of spermatogenesis probably contains 

the histogenetic nucleoplasm ........ 376 

IV. The Foregoing Considerations applied to Plants .... 377 

Z 



338 ON THE NUMBER OP POLAR BODIES, &C. — CONTENTS. 

PAOB 

V. Conclusions as begabds Hebeditt 378 

The germ-cell of an individual contains an unequal combination of 

hereditary tendencies . 378 

Dissimilarity between the offsprinj? of the same parents . . -379 
Identity of twins produced from a single egg 380 

VI. Rkcapitdlation 383 



VI. 



ON THE NUMBER OF POLAR BODIES AND THEIR 
SIGNIFICANCE IN HEREDITY. 

I. Parthenogenetic and Sexual Egg. 

Hitherto no value has been attached to the question whether an 
animal egg produces one or two polar bodies. Several observers 
have found two such bodies in many different groups of animals, 
both high and low in the scale of organization. In certain species 
only one has been observed, in others again three, four, or five (e. g. 
Bischoff, in the rabbit). Many observers did not even record the 
number of polar bodies found by them, and simply spoke of ' polar 
bodies.' As long as their formation was looked upon as a process 
of secondary physiological importance — as an ' excretion,' or a 
' process of purification,' or even as the ' excreta ' (!) of the egg, as 
a ' rejuvenescence of the nucleus,' or of mere historical interest 
as a reminiscence of ancestral processes, without any present 
physiological meaning — so long was it unnecessary to attach any 
importance to the number of these bodies, or to pay special atten- 
tion to them. Of all the above-mentioned views, the one which 
explained polar bodies as a mere reminiscence of ancestral processes 
seemed to be especially well founded. Ten years ago we were far 
from being able to prove that polar bodies occurred in all animal 
eggs, and even in 1880, Balfour said in his excellent ' Comparative 
Embryology,' * It is very possible, not to say probable, that such 
changes [the formation of polar bodies] are universal in the animal 
kingdom, but the present state of our knowledge does not justify 
us in saying so^.' 

Even at the present day we are not, strictly speaking, justified in 
making this assertion, for polar bodies have not yet been proved to 
occur in certain groups of animals, such as reptiles and birds ; but 
they have been detected in the great majority of the large groups 
of the animal kingdom, and wherever they have been looked for 

' Vol. I. p. 60. 
z 2 



340 ON THE NUMBEB OF POLAB BOPIES AND 

with the aid of our modem highly efficient appliances, they have 
been found ^. 

A deeper insight into the process of fertilization has above all 
led to a closer study of antecedent phenomena. 

O. Hertwig^ and FoP showed that the formation of polar bodies 
was connected with a division of the nuclear substance of the egg. 
Hertwig and Biitschli* then proved that the body expelled from 
the egg possessed the nature of a cell, and thus led the way to the 
view that the formation of polar bodies is a process of cell-division, 
although a very unequal one. Even then there was no reason for 
attaching any special importance to the number of these bodies ; 
nor should we have such a reason if we agreed with Minot ^, Bal- 
fom* ^, and van Beneden in ascribing a high physiological signifi- 
cance to this process, and assumed that the expelled polar body 
is the male part of the previously hermaphrodite egg-cell. We 
should not know in what proportion the quantities of the ' male * 
and ' female ' parts were present, and it would therefore be impossible 
to decide, a priori, whether the ' male ' part had to be removed 
from the body of the egg-cell in one, two, or more portions. 

Even after the view that the nuclear substance is the essential 
element in fertilization had gained ground — a view chiefly due to 
Strasburger's investigations on the process of fertilization in Pha- 
nerogams — and after Hertwig's opinion had been confirmed, that 
the process of fertilization is essentially the conjugation of nuclei, 
even then there appeared to be no reason why the number of 
divisions undergone by the nucleus of the matm-e egg should be 
looked upon as an essential feature. 

* The most recent example of this kind is afforded by the excellent work of 
O. Schultze, 'Ueber die Reifung und Befruchtung des Amphibieneies,' Zeitschr. f. 
wiss. Zool., Bd. XLV. 1887. Schultze has proved that two polar bodies are expelled 
from the egg of the Axolotl and of the frog, although all previous observers, including 
O. Hertwig, had been unable to find them. Thus the latter authority states as the 
result of an investigation specially directed towards this point, that the nucleus ia 
transformed in a peculiar manner (' Befruchtung des thierischen Eies,' III. p. 81). 

•' O. Hertwig, ' Beitrage zur Kenntniss der Bildung, Befruchtung, und Theilung 
des thierischen Eies,' Morpholog. Jahrbuch, I, II, and III. 1875-77. 

^ H. Fol, ' Recherches sur la fecondation et le commencement de I'h^nogenie chez 
divers animaux.' Genfeve, Bsile, Lyon, 1879. 

* Biitschli, ' Entwicklungsgeschichtliche Beitrage,' Zeitschr. f. wiss. Zool. Bd. 
XXIX. p. 237. 1877. 

* C. S. Minot, ' Account, etc.' Proceedings Boston Soc. Nat. Hist., vol. xix. p. 165. 
1877. 

* F. M. Balfour, 'Comparative Embryology.* 



THEIR SIGNIFICANCE IN HEREDITY. 341 

This was the state of the subject at the time when I first made 
an attempt to ascertain the meaning- of the formation of polar 
bodies. I based my views upon the idea, which was just then 
gaining ground, that Nageli's idioplasm was to be sought for in 
the nucleus, and that the nucleoplasm must therefore contain the 
substance which determines the form and functions of the cell. 
Hence it followed that the germ-plasm — the substance which de- 
termines the course of embryonic development — must be identified 
with the nucleoplasm of the egg-cell. The conception of germ- 
plasm was brought forward by me before the appearance of Nageli's 
work ^ which is so rich in fertile ideas ; and germ-plasm does not 
exactly coincide with Nageli's idioplasm ^. Germ-plasm is only 
a certain kind of idioplasm — viz. that contained in the germ-cell — 
and it is the most important of all idioplasms, because all the other 
kinds are merely the results of the various ontogenetic stages into 
which it developes. I attempted to show that the molecular 
structure in these ontogenetic stages into which the germ-plasm 
developes would become more and more unlike that of the original 
structure of this substance, until it finally attains a highly 
specialized character at the end of embryonic development, corre- 
sponding to the production of specialized histological elements. 
It did not seem to me to be conceivable that the specialized idio- 
plasm contained in the nuclei of the tissue cells could re-transform 
itself into the initial stage of the whole developmental series — 
that it could give up its specialized character and re-assume the 
generalized character of germ- substance. I will not repeat the 
reasons which induced me to adopt this opinion ; they still seem to 
me to be conclusive. But let the above-mentioned theory be once 
accepted, and there follows from it another interesting conclusion 
concerning the germ-cell, or at least concerning those germ-cells 
which, like most animal eggs, possess a specific histological cha- 
racter. For obviously, such a character presupposes the existence 
of an idioplasm with a considerable degree of histological special- 
ization, which must be contained in the nucleus of the egg-cell. 
We know, on the other hand, that when its growth is complete, 
after the formation of yolk and membranes, the egg contains 

* Nageli, ' Mechanisch-physiologische Theorie der Abstammungslehre,' Mfinchen 
und Leipzig, 1 884. 

* See the second and fourth Essays in the present volume. 



342 ON THE NUMBliR OF POLAR BODIES AND 

germ-plasm, for it is capable of developing into an embryo. We 
have therefore, as it were, two natures in a single cell, which 
become manifest one after the other, and which, according to our 
fundamental conception, caj only be explained by the presence of 
two different idioplasms, which control the egg-cell one after the 
other, and determine its processes of development. At first a 
nucleoplasm leading to histological specialization directs the de- 
velopment of the egg and stamps upon it a specific histological 
character ; and then germ-plasm takes its place, and compels the 
Ggg to undergo development into an embryo. If then the histo- 
genetic or ovoganetic nucleoplasm of the egg-cell can be derived 
from the germ-plasm, but cannot be re-transformed into it (for the 
specialized can be derived from the generalized, but not the 
generalized from the specialized), we are driven to the conclusion 
that the germ-plasm, which is already present in the youngest 
egg-cell, first of all originates a specific histogenetic or ovogenetic 
nucleoplasm which controls the egg-cell up to the point at which 
it becomes mature ; that its place is then taken by the rest of the 
unchanged nvacleoplasm (germ-plasm), which has in the meantime 
increased by growth ; and that the former is removed from the 
egg in the form of polar bodies — a removal which has been ren- 
dered possible by the occurrence of nuclear division. Hence the 
formation of polar bodies signified, in my opinion, the removal of 
the ovogenetic part of the nucleus from the mature egg-cell. Such 
removal was absolutely necessary, if it is impossible that the ovo- 
genetic nucleoplasm can be re-transformed into germ-plasm. Hence 
the former substance cannot be made use of after the maturation 
of the egg, and it must even be opposed to the commencement of 
embryonic development, for it is impossible that the egg can be 
controlled by two forces of different kinds in the same manner as 
it would have been by one of them alone. I therefore concluded 
that the influence of the ovogenetic idioplasm must be removed 
before embryonic development can take place. In this way it 
seemed to me that not only the ordinary cases of ovogenetic and em- 
bryonic development became more easily intelligible, but also the 
rarer cases in which one and the same species produces two kinds 
of eggs — ' summer and winter eggs.' Such eggs not only differ 
in size but also in the structure of yolk and membranes, although 
identical animals are developed from each of them. This result pre- 



THEIR SIGNIFICANCE IN HEREDITY. 843 

supposes that the nucleus in both eggs contains identical germ- 
plasm, while the formation of different yolks and membranes re- 
quires the supposition that their nucleoplasm is different, inasmuch 
as the two eggs differ greatly in histological character. 

The fact that equal quantities are separated during nuclear 
division, led me to conclude further that the expulsion of ovogenetic 
nucleoplasm can only take place when the germ-plasm in the 
nucleus of the egg-cell has increased by growth up to a point at 
which it can successfully oppose the ovogenetic nuclear substance. 
But we do not know the proportion which must obtain between the 
relative quantities of two different nuclear substances in order that 
nuclear division may be induced ; and thus, by this hypothesis at 
least, we could not conclude with certainty as to the necessity for 
a single or a double division of the egg. It did not seem to be 
altogether inconceivable that the ovogenetic nucleoplasm might be 
larger in amount than the germ-plasm, and that it could only be 
completely removed by means of two successive nuclear divisions. 
I admit that this supposition caused me some uneasiness ; but since 
nothing was known which could have enabled us to penetrate more 
deeply into the problem, I was satisfied, for the time being, in 
having found any explanation of the physiological value of polar 
bodies ; leaving the future to decide not only whether such explana- 
tion were valid, but also whether it were exhaustive. The explana- 
tion seems to have found but little favour with some of our 
highest authorities. Hensen ^ does not consider that my reasons for 
the distinction between germ-plasm and histogenetic nucleoplasm 
are conclusive, and it may be conceded that this objection was 
perhaps, at that time, well founded. O. Hertwig does not mention 
my hypothesis at all in his work on embryology^, although he 
states in the preface : ' Among current problems I have chiefly 
taken into consideration the views which seem to me to be most 
completely justified, but I have not left unmentioned the views 
which I cannot accept.' Minot's hypothesis is discussed by Hert- 
wig, but Biitschli's^ is preferred by him, although these two 

^ Hensen, ' Die Grundlagen der Vererbung,' Zeitsclir. f. wiss. Landwirthachaft. 
Berlin, 1885, p. 749. 

^ 0. Hertwig, ' Lehrbuch der Entwicklungsgeschichte des Menschen und der Wir- 
belthiere.' Jena, 1886. 

^ Biitschli, * Gedanken iiber die morphologische Bedeutung der BOg. Eichtungskor- 
perchen,* Biol. Centralblatt, Bd. VI. p. 5. 1884. 



344 ON THE NUMBER OF POLAB BODIES AND 

hypotheses are not sti'ictly opposed to each other ; for the former 
is a purely physiological, the latter a purely morpholog-ical ex- 
planation. I desire to lay especial stress upon the fact that my 
hypothesis is simply a logical consequence from the conclusion that 
the nuclear substance determines the nature of a cell. How this 
takes place is quite another question, which need not be discussed 
here. If it is only certain that the nature of a cell is thus deter- 
mined, it follows that a cell with a certain degree of histological 
specialization must contain a nucleoplasm corresponding to the 
specialization. But the mature egg also contains germ-plasm, and 
there are only two possibilities by which these facts can be 
explained : either the ovogenetic nucleoplasm is capable of re- 
transformation into germ-plasm, or it is incapable of such re-trans- 
formation. Now, quite apart from the arguments which might be 
advanced in favour of one of these two possibilities, the fact that 
a body is undoubtedly expelled from the mature egg seems to me 
of importance, while it is of even greater importance that this body 
contains nucleoplasm from the germ-cell. 

It may be thought that the process, as supposed by me, is 
without analogy, but such a conclusion is wrong, for dxiring every 
embryonic development there are numerous cell-divisions in which 
imequal nucleoplasms are separated from one another, and in all 
these cases we cannot imagine any way in which the process can 
take place, except by supposing that the two kinds of nucleoplasm 
were previously united in the mother-cell, although their differ- 
entiation probably took place only a short time before cell-division. 
Perhaps the new facts which will be mentioned presently, and the 
views derived from them, will make my hypothesis upon the histo- 
genetic nucleoplasm of the germ-cells appear in a more favourable 
light to the authorities above-named. 

My hypothesis has at all events the one merit that it has led 
me to fruitful investigations. 

If the formation of polar bodies really means the removal of ovo- 
genetic nucleoplasm from the mature egg, they must also be found 
in parthenogenetic eggs ; inasmuch as the latter possess a specific 
histological structure equal to that found in eggs requiring fertiliza- 
tion. If, therefore, it were possible to observe the formation of polar 
bodies in eggs which develope parthenogenetically, such an observa- 
tion would not form a proof of the validity of my interpretation ; but 



THEIR SIGNIFICANCE^ IN HEREDITY. 345 

it would be a fact which harmonized with it, and neg-atived a 
suggestion which, if confirmed, would have been fatal to the hypo- 
thesis. Minot, Balfour, and van Beneden, from the point of view 
afforded by their theories, were compelled to suppose that polar 
bodies are wanting in parthenogenetic eggs ; and the facts which 
were known at that time favoured such an opinion, for in spite 
of many attempts, no one had ever succeeded in proving the 
formation of these bodies by parthenogenetic eggs. 

During the summer of 1885 I first succeeded in ascertaining 
that a single polar body is expelled from the parthenogenetic 
summer-egg of one of the Bajphnidae^ — Polyphemus ocuhs'^. Thus 
my interpretation of the process in question received support, while 
it seemed to me that Minot's interpretation of polar bodies had 
been refuted ; for if these bodies are formed in the parthenogenetic 
eggs of a single species, just as in eggs which require fertilization, 
it follows that the expulsion of polar bodies cannot signify the 
removal of the male element from the eg^. 

The desire to throw light upon the significance of polar bodies 
has been the only cause of my investigation. At the same time 
I hoped by this means to gain further knowledge as to the nature 
of parthenogenesis. 

In the third part of the essay on ' The Continuity of the Germ- 
plasm' (see p. 325) I attempted to make clear the nature of 
parthenogenesis, and I arrived at the conclusion that the difference 
between an egg which is capable of developing without fertilization, 
and another which requires fertilization, must lie in the quantity 
of nucleoplasm present in the egg. I supposed that the nucleus of 
the mature parthenogenetic Qgg contained nearly twice as much 
germ-plasm as that contained in the sexual egg, just before the 
occurrence of fertilization ; or, more correctly, I believed that the 
quantity of nucleoplasm which remains in the Qgg, after the ex- 
pulsion of the polar bodies, is the same in both eggs, but that the 
parthenogenetic egg possesses the power of doubling this quantity 
by growth, and thus produces from within itself the same quantity 
of germ -plasm as that contained in the sexual egg after the 
addition of the sperm-nucleus in fertilization. 

This was only an hypothesis, and the considerations which had led 

* This observation was first published as a note at the end of the fourth Essay in 
the present volume. See p. 249. 



346 ON THE NUMBEB OP POLAR BODIES AND 

to it depended, as far as they went into details, upon assumptions ; 
but the fundamental view that the quantity of the nucleus decides 
whether embryonic development takes place with or without fer- 
tilization seemed to me, even at that time, to be correct, and to be 
a conclusion required by the facts of the case. Indeed, I thought 
it not unlikely that its validity might be proved by direct means : 
I pointed out that a comparison of the quantities of the nuclei in 
parthenogenetie and sexual eggs, if possible in the same species, 
would enable us to decide the question {I. c, p. 234). 

I had thus set myself the task of making this comparison. The 
result of this investigation was to show that, as already mentioned, 
polar bodies are formed in parthenogenetic eggs. But even the 
first species successfully investigated revealed a further fact, which, 
if proved to be wide-spread and characteristic of all partheno- 
genetic eggs, was certain to be of extreme importance : — the matura- 
tion of the parthenogenetic egg is accompanied by the expulsion of 
one polar body, or, as we might express it in another way, the 
substance of the female pronucleus is only 07ice divided, and not 
twice, as in the sexual eggs of so many other animals. If this 
difference between parthenogenetic and sexual eggs was shown to 
be general, then the foundations of my hypothesis would indeed 
have been proved to be sound. The quantity of nuclear substance 
decides whether the egg is capable of undergoing embryonic 
development. This quantity is twice as large in the partheno- 
genetic as in the sexual egg. I had, however, been mistaken in 
a matter of detail ; for the difference in the quantities of nuclear 
substance is not produced by the expulsion of two polar bodies, and 
the reduction of the nuclear substance to a quarter of- its original 
amount, in both eggs, while the parthenogenetic egg then doubles 
its nuclear substance by growth ; but the difference is produced 
because the reduction of nuclear substance originally present is less 
in one case than it is in the other. In the parthenogenetic egg 
the nuclear substance is only reduced to one-half by a single 
division ; in the sexual egg it is reduced to a quarter by two 
successive divisions. It is an ob\dous conclusion from this fact, if 
proved to be wide-spread, that the significance of the first polar 
body must be different from that of the second. Only one polar 
body can signify the removal of ovogenetic nucleoplasm from the 
mature egg, and the second is obviously a reduction of the germ- 



THEIK SIGNIFICANCE IN HEREDITY. 347 

plasm itself to half of its original amount. This very point seemed 
to me to be of great importance, because, as I had foreseen long 
ago, and as will be shown later on, the theory of heredity forces us 
to suppose that every fertilization must be preceded by a reduction 
of the ancestral idioplasms present in the nucleus of the parent 
germ-cell, to one-half of their former number. 

But before the full bearing of the phenomena could be considered, 
it was necessary to ascertain how far they were of general occur- 
rence. There were two ways in which this might be achieved, and 
in which it was possible to prove that parthenogenetic eggs expel 
only one polar body, while sexual eggs expel two. We might 
attempt to observe the phenomena of maturation in both kinds of 
eggs in a species which reproduces itself by the parthenogenetic 
as well as the sexual method. This would be the simplest way in 
which the question could be decided, if it were possible to make 
such observations on a sufficient number of species. But the other 
method was also open, a method which would have been the only 
one, if we did not know of any animals with two kinds of repro- 
duction. We might attempt to investigate the phenomena of 
maturation in a large number of parthenogenetic eggs, if possible 
from different groups of animals, and we might compare the results 
with the facts which are already certain concerning the expul- 
sion of polar bodies from the sexual eggs of so many species. 

I have followed both methods, and by means of the second 
I have arrived already, indeed some time ago, at the certain con- 
clusion that the above-mentioned difference is really general and 
without exception. The first polar body only is formed in all the 
parthenogenetic eggs which I investigated, with the valuable 
assistance of my pupil, Mr. Ischikawa of Tokio. On the other 
hand, an extensive examination of the literature of the subject 
convinced me that there is not a single undoubted instance of the 
expulsion of only one polar body from eggs which require fertiliza- 
tion, and that there are very numerous cases known from almost all 
groups of the animal kingdom in which it is perfectly certain that 
two polar bodies are formed, one after the other. A number of the 
older observations cannot be relied upon, for the presence of two 
polar bodies is mentioned without any explanation as to whether 
they are expelled from the egg one after the other, or whether 
they have merely resulted from the division of a single body after 



348 ON THE NUMBER OP POLAR BODIES AND 

its expulsion. In parthenogenetic eggs two polar bodies are also 
formed in most cases, but they arise from the subsequent division 
of the single body which separates from the egg. But such sub- 
sequent division is only of secondary importance as far as the egg 
itself is concerned, and is also unimportant in the interpretation of 
the process. The essential nature of the process is to be found in 
the fact that the nucleus of the egg-cell only divides once when 
parthenogenesis occurs, but twice when fertilization is necessary, and 
it is of no importance whether the expelled part of the nucleus of 
the cell-body atrophies at once, or after it has undergone division. 
"We have, therefore, to distinguish between primary and secondary 
polar bodies. If this distinction is recognized, and if we leave out 
of consideration all doubtful cases mentioned in literature, such a 
large number of well- established observations remain, that the 
existence of two primary polar bodies in sexual eggs, and neither 
a smaller nor a larger number, may be considered as proved. 

Hence follows a conclusion which I believe to be very significant, 
— the difference between parthenogenetic and sexual eggs lies in 
the fact that in the former only one primary polar body is ex- 
pelled, while two are expelled from the latter. When, in July, 
1886, I published a short note ^ on part of the observations made 
upon parthenogenetic eggs, I confined myself to facts, and did not 
mention this conclusion. I took this course simply because I did 
not wish to bring it forward until I had made sufiicient observa- 
tions in the first of the two ways described above. I had hoped 
to be able to ofier all the proofs that can be obtained before 
undertaking to publish the far-reaching consequences which would 
result from the above-mentioned conclusion. Unfortunately the 
material with which I had hoped to quickly settle the matter, 
proved less favourable than I had expected. Many hundred 
sections through freshly laid winter-eggs of Bythotrephes longi- 
matms were made in vain ; they did not yield the wished for 
evidence, and although continued investigation of other material 
has led to better results, the proofs are not yet entirely com- 
plete. 

I should not therefore even now have brought forward the above- 
mentioned conclusion, if another observer had not alluded to this 

* Weismann, ' Bicbtungskorper bei partbenogenetiscben Eieren,' Zool. Anzeiger, 
1886, p. 570. 



THEIR SiaNIFICANCE IN HEREDITY. 349 

idea, referring* to my observations and also to a new discovery of 
his own. In a recent number of the ' Biologische Centralblatt,' 
Blochmann ^ gives an account of his continued observations upon 
the formation of polar bodies. It is well known that this careful 
observer had previously shown that polar bodies do occur in the 
egg-s of insects, although they had not been found before. Bloch- 
mann proved that they are found in the representatives of three 
different orders, so that we may indeed ' confidently hope to find 
corresponding phenomena in other insects.' This discovery is 
most important, and it was naturally very welcome to me, as I 
had for a long time ascribed a high physiological importance 
to the process of the formation of polar bodies, and it would not 
be in accordance with such a view if the process was entirely 
wanting from whole classes of animals. To fill up this gap in 
our knowledge, and to give the required support to my theoretical 
viewSj I had proposed to one of my pupils. Dr. Stuhlmann^, that 
he should work, out the matm'ation of the eggs of insects ; and it 
is a curious ill-luck that he, like many other observers, did not 
succeed in observing the expected expulsion of polar bodies, in 
spite of the great trouble he had taken. It may be that the 
species selected for investigation were unfavourable : at all events, 
we cannot now doubt that a division of the egg-nucleus is 
quite universal among insects, for Blochmann, in his latest con- 
tribution to the subject, proves that the ApJiidae also form polar 
bodies. He examined the winter-eggs of Ajohis aceris, and as- 
certained that they form two polar bodies, one after the other. 
Even in the viviparous Aphidae, thin sections revealed the presence 
of a polar body, though Blochmann could not trace all the stages 
of its development. It appears that the polar body is here pre- 
served for an exceptional period, and its presence can still be 
proved when the blastoderm has been formed, and sometimes 
when development is even further advanced. Skilled observers of 
recent times, such as Will and Witlaczil, have not been able to 
find a polar body in the parthenogenetic eggs of the Aphidae, and 

^ Bloclimann, 'Ueber die Eichtungskorper bei den Insekteneiern,' Biolog. Cen- 
tralblatt., April 15, 1887. 

^ F. StuMmann, 'Die Eeifung des Ai-thropodeneies nach Beobachtungen an 
Insekten, Spinnen, Myriapoden und Peripatus,' Berichte der naturforschenden 
Gesellschaft zu Freiburg i. Br., Bd. I. p. loi. 



350 ON THE NUMBER OF POLAR BODIES AND 

Blochmann's proof of its existence seems to me to be of especial 
value, because the eggs of Aphidae are in many respects so unusually 
reduced ; for instance, the primary yolk is absent and the egg- 
membrane is completely deficient, so that we might have expected 
that if polar bodies are ever absent, they would be wanting in these 
animals — that is, if they were of no importance, or at any rate of 
only secondary importance. 

Hence the presence of polar bodies in Aphidae is a fresh con- 
firmation of their great physiological importance. As bearing 
upon the main question dealt with in this essay, Blochmann's 
observations have an especial interest, because only one polar body 
was found in the parthenogenetic eggs of Aphis, while the sexual 
eggs normally produce two. The author rightly states that this 
result is in striking accordance with my results obtained from the 
summer-eggs of different Daphnidae, and he adds the remark, — ' It 
would be of great interest to know whether these facts are due to 
the operation of some general law.' To this reiqark I can now 
reply that there is indeed such a law : not only in the parthenogenetic 
eggs of Baphnidae, but also, as I have since found, in those of 
the Ostracoda and Rotifera ^, only one primary polar body is 
formed, while two are formed in all eggs destined for fertilization. 

Before proceeding to the conclusions which follow from this 
fact, I will at once remove a difficulty which is apparently pre- 
sented by the eggs which may develope with or without fertiliza- 
tion. I refer to the well-known case of the eggs of bees. It might 
be objected to my theory that the same egg cannot be prepared 
for development in more than one out of the two possible ways ; 
it might be argued that the egg either possesses the power of 
entering upon two successive nuclear divisions during maturation, 

* In the summer-eggs of Rotifera I have, together with Mr. Ischikawa, observed 
one polar body, and we were able to establish for certain that a second is not formed. 
The nuclear spindle had already been observed by Tessin, and Billet had noticed 
polar bodies in Philodina, but without attaching any importance to their number. 
These latter observations were not conclusive proofs of the formation of polar bodies 
in parthenogenetic eggs, so long as it was not known whether the summer-eggs of 
Rotifera may develope parthenogenetically, or whether they can only develope in 
this way. Knowing now that parthenogenetic eggs expel only one polar body, we 
may perhaps be permitted to draw the conclusion that the sunmier-egg of a Rotifer 
{Lacinularia) which expelled only one polar body must have been a parthenogenetic 
egg. But I may add that we have also succeeded in directly proving the occurrence 
of parthenogenesis in Rotifera, as will be described in detail in another paper. 



^ THEIR SIGNIFICANCE IN HEREDITY. 351 

and in this ease requires fertilization ; or the egg may be of such a 
nature that it can only enter upon one such division and can 
therefore form only one polar body, and in that case it is capable of 
parthenogenetic development. Now there is no doubt, as I pointed 
out in my paper on the nature of parthenogenesis ^, that in the bee 
the very same egg may develope parthenogenetically, which under 
other circumstances would have been fertilized. Bessel's " experi- 
ments, in which young queens were rendered incapable of flight, and 
were thus prevented from fertilization, have shown that all the eggs 
laid by such females develope into drones (males) which are well 
known to result from parthenogenetic development. On the other 
hand, bee-keepers have long known that young queens which are 
fertilized in a normal manner continue for a long time to lay eggs 
which develope into females, that is to say, which have been 
fertilized. Hence the same eggs, viz. those which are lowest in 
the oviducts and are therefore laid first, develope parthenogeneti- 
cally in the mutilated female, but are fertilized in the normal 
female. The question therefore arises as to the way in which the 
eggs become capable of adapting themselves to the expulsion of 
two polar bodies when they are to be fertilized, and of one only 
when fertilization does not take place. 

But perhaps the solution of this problem is not so difficult as it 
appears to be. If we may assume that in eggs which are capable 
of two kinds of development the second polar body is not expelled 
until the entrance of a spermatozoon has taken place, the explanation 
of the possibility of parthenogenetic development when fertilization 
does not occur would be forthcoming. Now we know, from the in- 
vestigations of O. Hertwig and Fol, that in the eggs of Echinus 
the two polar bodies are even formed in the ovary, and are therefore 
quite independent of fertilization, but in this and other similar cases 
a parthenogenetic development of the egg never takes place. There 
are, however, observations upon other animals which point to the fact 
that the first only and not the second polar body may be formed before 
the spermatozoon penetrates into the egg. It can be easily under- 
stood why it is that entirely conclusive observations are wanting, 
for hitherto there has been no reason for any accurate distinction 

^ See Essay IV, Part III. p. 225. 

* E. Bessels, ' Die Landoia'sche Theorie, widerlegt durch das Experiment.' 
Zeitschr. f. wiss. Zool. Bd. XVIII. p. 124. 1868. 



352 ON THE NUMBER OF POLAR BODIES AND 

between the first and the second polar body. Butin many eg^s it 
appears certain that the second polar body is not expelled until the 
spermatozoon has penetrated, O. Schultze, the latest observer of 
the egg' of the frog, in fact saw the first polar body alone extruded 
from the unfertilized egg : a second nuclear spindle was indeed 
formed, but the second polar body was not expelled until after 
fertilization had taken place. A very obvious theory therefore sug- 
gests itself: — that while the formation of the second polar body is 
purely a phenomenon of matm'ation in most animal eggs, and is 
independent of fertilization, — in the eggs of a number of other 
animals, on the other hand, and especially among Aithropods, 
the formation of the second nuclear spindle is the result of a 
stimulus due to the entrance of a spermatozoon. If this sug- 
gestion be confirmed, we should be able to understand why partheno- 
genesis occurs in certain classes of animals wherever the external 
conditions of life render its appearance advantageous, and further, 
why in so many species of insects a sporadic parthenogenesis is ob- 
served, viz. the parthenogenetic development of single eggs (Lepi- 
doptera). Slight individual differences in the facility with which 
the second nuclear spindle is formed independently of fertilization 
would in such cases decide whether an egg is or is not capable of 
parthenogenetic development. As soon, however, as the second 
nuclear spindle is formed, parthenogenesis becomes impossible. 
The nuclear spindle which gives rise to the second polar body, and 
that which initiates segmentation, axe two entirely different things, 
and although they contain the same quantity, and the same kind 
of germ-plasm, a transformation of the one into the other is 
scarcely conceivable. This conclusion will be demonstrated in the 
following part of the essay. 

II. The Significance of the. Second Polar Body. 

I have already discussed the physiological importance of the first 
polar body, or rather of the first division undergone by the nucleus 
of the egg, and I have explained it as the removal of ovogenetic 
nuclear substance which has become superfluous and indeed in- 
jurious after the maturation of the egg. I do not indeed know of 
any other meaning which can be ascribed to this process, now that 
we know of the occurrence of a first division of the nucleus in 



THEIR SIGNIFICANCE IN HEREDITY. 353 

parthenog-enetic as well as in sexual eggs. A part of the nucleus 
must thus be removed from both kinds of eggs, a part which was 
necessary to complete their growth, and which then became super- 
fluous and at the same time injurious. In this respect the observa- 
tions of Blochmann ^ upon the eggs of Musca vomitoria seem to me 
to be very interesting. Here the two successive divisions of the 
nuclear spindle arising from the egg-nucleus take place, but true 
polar bodies are not expelled, and the two nuclei corresponding to 
them (one of which divides once more) are placed on the sm-face of 
the Qggi surrounded by an area free from yolk granules ; and they 
break up at a later period. The essential point is obviously to 
eliminate from the egg-cell the influence of nucleoplasm which has 
been separated from the egg-nucleus as the first polar body; and 
this condition is satisfied whether the elimination is brought about 
by a process of true cell-division, as is the rule in the eggs of most 
animals, or by the division and removal of part of the egg-nucleus 
alone. The occurrence of the latter method of elimination cer- 
tainly constitutes a still further proof of the physiological im- 
portance of the process, and this, taken together with the uni- 
versal occurrence of polar bodies in all eggs — parthenogenetic and 
sexual — forces us to conclude that the process must possess a 
definite significance. No one of the various attempts which have 
been made to explain the significance of polar bodies generally 
is applicable to the Jirst polar body except that which I have 
attempted. 

But the case is different with the significance of the second 
nuclear division, or the second polar body. Here it might perhaps 
be possible to return to the view brought forward by Minot, 
Balfour, and van Beneden, and to consider the removal of this 
part of the nucleus as the expulsion of the male part of the pre- 
viously hermaphrodite egg-cell. The second polar body is only ex- 
pelled when the egg is to be fertilized, and at first sight it appears 
to be quite obvious that such a preparation of the egg for fertilization 
must depend upon its reduction to the female state. I believe how- 
ever that this is not the case, and am of opinion that the process 
has an entirely different and much deeper meaning. 

How can we gain any conception of this supposed herma- 
phroditism of the egg-cell, and its subsequent attainment of the 

^ 1. c, p. no. 
A a 



354 ON THE NUMBEB OP POLAR BODIES AND 

female state ? What are the essential characteristics of the male and 
female states ? We know of female and male individuals, among 
both animals and plants : their differences consist essentially in the 
fact that they produce different kinds of reproductive cells ; in part 
they are of a secondary nature, being adaptations of the organism 
to the functions of reproduction ; they are intended to attract the 
other sex, or to ensure the meeting of the two kinds of reproductive 
cells, or to enable the fertilized egg to develope and sometimes to 
guide the development of the offspring until it has reached a certain 
period of growth. But all these differences, however great they may 
sometimes be, do not alter the essential nature of the organism. 
The blood coi-puscles of man and woman are the same, and so are 
the cells of their nerves and muscles ; and even the sexual cells, so 
different in size, appearance, and generally also in motile power, 
must contain the same fundamental substance, the same idioplasm. 
Otherwise the female germ-cell could not transmit the male 
characters of the ancestors of the female quite as readily as the 
female characters, nor could the male germ-cell transmit the female 
quite as readily as the male characters of the ancestors of the male. 
It is therefore clear that the nuclear substance itself is not sexually 
differentiated. 

I have already previously pointed out that the above-mentioned 
facts of heredity contain the disproof of Minot's theory, inasmuch 
as the egg-cell transmits male as well as female characters. Stras- 
burger ^ has also raised a similar objection. I consider this objec- 
tion to be quite conclusive, for there does not seem to be any way 
in which the difficulty can be met by the supporters of the theory. 
The difficulty could indeed be evaded until we came to know that 
the essential part of the polar body is nuclear substance, and that the 
latter must be regarded as idioplasm, — as the substance which is the 
bearer of heredity. It might have been maintained that the male 
part, removed from the egg, consists only in a condition, perhaps 
comparable to positive or negative electricity ; and that this con- 
dition is present in the substance of the polar body, so that the 
removal of the latter would merely signify a removal of the 
unknown condition. I do not mean to imply that any of those 
who have adopted Minot's theory have had any such vague ideas 

* Strasburger, 'Neue Untersuchungen iiber den Befruchtungsvorgang bei den 
Phanerogamen als Grundlage einer Theorie der Zeugung.' Jena, 1884. 



THEIE SIGNIFICANCE IN HEKEDITY. 355 

concerning" this process, but even if any one were ready to adopt it, 
he would be unable to make any use of the idea. He would 
not be able to support the theory in this way, for we now know 
that nuclear substance is removed with the polar body, and this 
fact requires an explanation which cannot be afforded l)y the 
theory, if we are right in believing that the expelled nuclear sub- 
stance is not merely the indifferent bearer of the unknown principle 
of the male condition, but hereditary substance. I therefore be- 
lieve that Minot's, Balfour's, and van Beneden's hypothesis, al- 
though an ingenious attempt which was quite justified at the time 
when it originated, must be finally abandoned. 

My opinion of the significance of the second polar body is 
shortly this, — a reduction of the g-erm-plasm is brought about by its 
formation, a reduction not only in quantity, but above all in the 
complexity of its constitution. By means of the second nuclear 
division the excessive accumulation of different kinds of hereditary 
tendencies or germ-plasms is prevented, which without it would be 
necessarily produced by fertilization. With the nucleus of the 
second polar body as many different kinds of idioplasm are removed 
from the egg as will be afterwards introduced by the sperm- 
nucleus ; thus the second division of the egg-nucleus serves to 
keep constant the number of different kinds of idioplasm, of which 
the germ-plasm is composed during the course of generations. 

In order to make this intelligible a short explanation is necessary. 

From the splendid series of investigations on the process of 
fertilization, commenced by Auerbach and Butschli, and continued 
by Hertwig, Fol, Strasburger, van Beneden, and many others, 
and from the theoretical considerations brought forward by Pfliiger, 
Nageli, and myself, at least one certain result follows, viz. that 
there is an hereditary substance, a material bearer of hereditary 
tendencies, and that this substance is contained in the nucleus 
of the germ-cell, and in that part of it which forms the nuclear 
thread, which at certain periods appears in the form of loops or 
rods. We may further maintain that fertilization consists in the 
fact that an equal number of loops from either parent are placed 
side by side, and that the segmentation nucleus is composed in this 
way. It is of no importance, as far as this question is concerned, 
whether the loops of the two parents coalesce sooner or later, 
or whether they remain separate. The only essential conclusioji 

A a 2 



356 ON THE NUMBER OP POLAR BODIES AND 

demanded by our hypothesis is that there should be complete or 
approximate equality between the quantities of hereditary sub- 
stance derived from either parent. If then the germ-cells of the 
offspring contain the united germ-plasms of both parents, it follows 
that such cells can only contain half as much paternal germ-plasm 
as was contained in the germ-cells of the father, and half as much 
maternal germ-plasm as was contained in the germ-cells of the 
mother. This principle is affirmed in a well-known calculation 
made by breeders of animals, who only differ from us in their use of 
the term 'blood' instead of the term germ-plasm. Breeders say that 
half of the ' blood' of the offspring has been derived from the father 
and the other half from the mother. The grandchild similarly 
derives a quarter of its ' blood ' from each of the four grandparents, 
and so on. 

Let us imagine, for the sake of argument, that sexual repro- 
duction had not been introduced into the animal kingdom, and 
that asexual reproduction had hitherto existed alone. In such a 
case, the germ-plasm of the first generation of a species which 
enters upon sexual reproduction must still be entirely homo- 
geneous ; the hereditary substance must, in each individual, con- 
sist of many minute units, each of which is exactly like the other, 
and each of which contains within itself the tendency to transmit, 
under certain circumstances, the whole of the characters of the 
parent to a new organism — the offspring. In each of the offspring 
of such a first generation, the germ-plasms of two parents will be 
united, and every germ-cell contained in the individuals of this second 
sexually produced generation will now contain two kinds of germ- 
plasm — one kind from the father, and the other from the mother. 
But if the total quantity of germ-plasm present in each cell is to 
be kept within the pre-determined limits, each of the two ancestral 
germ-plasms, as I may now call them, must be represented by only 
half as many units as were contained in the parent germ-cells. 

In the third sexually produced generation, two new ancestral 
germ-plasms would be added by* fertilization to the two already 
present, and the germ-cells of this generation would therefore con- 
tain four different ancestral germ-plasms, each of which would 
constitute a quarter of the total quantity. In each succeeding 
generation the number of the ancestral germ-plasms is doubled, 
while their quantities are reduced by one half Thus in the fifth 



THEIE SIGNIFICANCE IN HEKEDITY. 357 

sexually produced g-eneration, each of the sixteen ancestral germ- 
plasms will only constitute yV of the total quantity; in the sixth, 
each of the thirty-two ancestral germ-plasms, only -^^, and so on. 
The germ-plasm of the tenth generation would be composed of 
1024 different ancestral germ-plasms, and that of the n*^ of 2^. 
By the tenth generation each single ancestral germ-plasm would 
only form xoVt of the total quantity of germ-plasm contained in 
a single germ-cell. We know nothing whatever of the length of 
time over which this process of division of the ancestral germ- 
plasms may have endured, but even if it had continued to the 
utmost possible limit — so far indeed that each ancestral germ- 
plasm was only represented by a single unit — a time would at last 
come when any further division into halves would cease to be 
possible ; for the very conception of a unit implies that it cannot 
be divided without the loss of its essential nature, which in this 
case constitutes it as the hereditary substance. 

In the diagram represented in Kg. I. I have tried to render 
these conclusions intelligible. In generation i. each paternal 
and maternal germ-plasm is still entirely homogeneous, and does 
not contain any combination of different hereditary qualities, but 
the germ- plasm of the offspring is made up of equal parts of 
two kinds of germ-plasm. In the second generation this latter 
germ-plasm unites with another derived from other parents, which 
is similarly composed of two ancestral germ-plasms, and the re- 
sulting third generation now contains four different ancestral germ- 
plasms in its germ-cells, and so on. The diagram only indicates the 
fusion of ancestral germ-plasms as far as the offspring of the fourth 
generation, the germ-cells of which contain sixteen different an- 
cestral germ-plasms. If we imagine the germ-plasm units to be 
so large that there is only room for sixteen of them in the nuclear 
thread, the limits of division would ~be reached in the fifth genera- 
tion, and any further division into halves of the ancestral germ- 
plasms would be impossible. 

Now however minute the units may be, there is not the least 
doubt that the limits of possible division have been long since 
reached by all existing spedies, for we may safely assume that no 
one of them has aequii'ed the sexual method of reproduction within 
a small number of recent generations. All existing species must 
therefore now contain as many different kinds of ancestral germ- 



358 



ON THE NUMBER OF POLAR BODIES AND 



plasms as they are capable of containing ; and the question arises, — 
How can sexual reproduction now proceed without a doubling of 
the quantity of germ-plasm in each germ-cell, with every new 
generation ? 

There is only one possible answer to such a question : — sexual re- 
production can proceed by a reduction in the number of ancestral 
germ-plasms, a reduction which is repeated in every generation. 

Father. Mother. Offspring. 








Generation II. 



Fig. I. 

This must be so : the only question is, how and when does the 
supposed reduction take place. 

Inasmuch as the germ-plasm is seated, according to our theory, 
in the nucleus, the necessary reduction can only be produced by 
nuclear division ; and quite apart from any observation which has 
been already made, we may safely assert that there must be a form 
of nuclear division in which the ancestral germ-plasms contained in 



THEIR SIGNIFICANCE IN HEREDITY. 359 

the nucleus are distributed to the daughter-nuclei in such a way 
that each of them receives only half the number contained in the 
original nucleus. After Roitx's ^ elaborate review of the whole 
subject, we need no longer doubt that the complex method of 
nuclear division, hitherto known as karyokinesis, must be con- 
sidered not merely as a means for the division of the total quantity 
of nuclear substance, but also for producing a division of the 
quantity and quality of each of its single elements. In by far the 
greater number of instances the object of this division is obviously 
to effect an equal distribution of nuclear substance in the two 
daughter-nuclei, so that each of the different qualities contained in 
the mother-nucleus is transferred to the two daughter-nuclei. This 
interpretation of ordinary karyokinesis is less uncertain than per- 
haps at first sight it may appear to be. We cannot, it is true, 
directly see the ancestral germ-plasms, nor do we even know the 
parts of the nucleus which are to be looked upon as constituting 
ancestral germ -plasm ; but if Flemming's original discovery of the 
longitudinal division of the loops lying in the equatorial plane of 
the nuclear spindle is to have any meaning at all, its object must 
be to divide and distribute the different kinds of the minutest 
elements of the nuclear thread as equally as possible. It has been 
ascertained that the two halves produced by the longitudinal split- 
ting of each loop never pass into the same daughter-nucleus, but 
always in opposite directions. The essential point cannot therefore 
be the division of the nucleus into absolutely equal quantities, but it 
must be the distribution of the different qualities of the nuclear 
thread, without exception, in both daughter-nuclei. But these dif- 
ferent qualities are what I have called the ancestral germ-plasms, i.e. 
the germ-plasms of the different ancestors, which must be contained 
in vast numbers, but in very minute quantities, in the nuclear thread. 
The supposition of a vast number is not only required by the 
phenomena of heredity but also results from the comparatively 
great length of the nuclear thread : furthermore it implies that 
each of them is present in very small quantity. The vast number 
together with the minute quantity of the ancestral germ-plasms 
permit us to conclude that they are, upon the whole, arranged in a 
linear manner in the thin thread-like loops : in fact the longitudinal 

' Wilhelm Roux, ' Ueber die Bedeutung der Kerntheilungsfiguren.' Leipzig, 
1884. 



860 ON THE NUMBER OF POLAR BODIES AND 

splitting of these loops appears to me to be almost a proof of the 
existence of such an arrangement, for without this supposition the 
process would cease to have any meaning. 

This is the only kind of karyokinesis which has been observed 
until recently; but if the supposed nuclear division leading to 
a reduction in the number of ancestral germ-plasms has any real 
existence, there must be yet another kind of karyokinesis, in 
which the primary equatorial loops are not split longitudinally, but 
are separated without division into two groups, each of which forms 
one of the two daughter-nuclei. In such a case the required reduc- 
tion in the number of ancestral germ-plasms would take place, for 
each daughter-nucleus would receive only half the number which 
was contained in the mother-nucleus. 

Now there is more evidence for the existence of this second kind 
of karyokinesis than the fact that it is demanded by my theory ; 
for I believe that it has been abeady observed, although it has not 
been interpreted in this sense. 

It is very probable that this is true of van Beneden's ^ observation 
on the egg of Ascaris megalocephala : he found that the nuclear 
division which led to the formation of the polar body diflFers from 
the ordinary course of karyokinesis, in that the plane of division 
is at right angles to that usually assumed. Camoy ^ has confirmed 
this obsei"vation in its main features, and he has made the further 
observation that out of the eight nuclear loops which are found at 
the equator of the spindle, four are removed with the first polar 
body, and that half of the remaining four are removed with the 
second polar body. The first of these two divisions would hare to 
be looked upon as a reduction, if it is certain that each of the eight 
nuclear loops consists of different ancestral germ-plasms ; but this 
assumption is impossible, although on the other hand it cannot be 
directly disproved : for we are not able to see the ancestral germ- 
plasms. But it must nevertheless be maintained that the removal 
of the first four loops does not imply a reduction in the number of 
ancestral germ-plasms in the nucleus ; because, as I have already 
ai'gued, two successive divisions of the number of ancestral germ- 

• E. van Beneden, ' Recherches sur la maturation de I'oeuf, la fecondation et la 
division cellulaire.' Gand et Leipzig, Paris, 1883. 

^ J. B. Camoy, ' La Cytodierfese de I'oeuf, la v^sicule genninative et les globules 
polaires de I'Ascaris megalocephala.' Louvain, Gand, Lierre, 1886. 



THEIK SIGNIFICANCE IN HEREDITY. 361 

plasms into halves is inconceivable ; and because the first polar 
body is also present in parthenogenetic eg-gs in which such division 
into halves cannot take place. But the karyokinetic process can 
readily be looked upon as a removal of ovogenetic nucleoplasm, for 
we know from the observations of Flemming and Carnoy, that, 
under certain circumstances, subsequent divisions may occur, in- 
volving an increase in the number of nuclear loops to double their 
number. These subsequent divisions of course take place in the 
daughter-nuclei. This fact proves, as I think, that there are nuclei 
in which the same ancestral germ-plasm occurs in two different 
loops : but such loops, identical as regards the composition of their 
ancestral germ-plasms, may very well contain different ontogenetic 
stages of this substance. This will be the case in the instance 
alluded to, if four loops of the first nuclear spindle are to be looked 
upon as ovogenetic nucleoplasm, and the four others as germ- 
plasm. It is therefore unnecessary to regard the first division of 
the egg-nucleus as a ' reducing division ' : it may be looked upon as 
an 'equal division ' ^ entirely analogous to the kind of division which, 
in my opinion, directs the development of the embryo. This con- 
clusion would receive direct proof if it were possible to show that 
the eight loops of the first division have arisen by the longitudinal 
splitting of foiir primary loops : for a longitudinal splitting of the 
nuclear thread would be the means by which the different onto- 
genetic stages of the germ-plasm could be separated from one 
another, without leading to any reduction in bhe number of ances- 
tral germ-plasms in the daughter-nuclei. Thus I have previously 
attempted to prove that the ontogenetic development of the o^^^ 
must be connected with a progressive transformation of the nucleo- 
plasm during successive nuclear divisions, and this transformation 
will very frequently (but not always) occur in such a way that the 
different qualities of the nucleoplasm are separated from one another 
by the nuclear division. The nucleoplasm of the daughter-nuclei 
will be identical if the two daughter-cells are to potentially contain 
corresponding parts of the embryo ; as for instance the first two 
segmentation spheres of the ^^^ of the frog, which according to 
Roux^ correspond to the right and left halves of the future animal. 

^ See p. 364. 

* Wilhelm Roux, 'Beitrage zur Entwicklungsmechanik des Embryo,' No. 3, 
Breslauer arztliche Zeitschrift, 1885, p. 45. 



362 ON THE NUMBER OF POLAR BODIES AND 

But the nucleoplasm must be unequal if the products of division 
are to develope into different parts of the embryo. In both cases, 
however, karyokinesis is connected with a longitudinal splitting of 
the nuclear threads, and we may conclude from this fact (which is 
also confirmed by the phenomena of heredity) that all such nuclei, 
whether they have entered upon the same or different ontogenetic 
transformations of their nucleoplasm, are identical as regards the 
ancestral germ-plasm which they contain. During the whole pro- 
cess of segmentation and the entire development of the embryo, the 
total number of ancestral germ-plasms which were at first contained 
in the germ-plasm of the fertilized egg-cell must still be contained 
in each of the succeeding cells. 

Thus no objection can be raised against the view that the four 
loops of the first polar body contain the ovogenetic nucleoplasm, 
that is to say, an idioplasm which contains the total number of an- 
cestral germ-plasms, but at an advanced and highly specialized 
ontogenetic stage. 

The formation of the second polar body may be rightly considered 
as a ' reducing division,' as a division leading to the expulsion of 
half the number of the different ancestral germ-plasms, in the form 
of two nuclear loops, for no reason can be alleged in support of the 
assumption that the four loops of the second nuclear spindle are 
made up of identical pairs. Furthermore the &cts of heredity re- 
quire the assumption that the greatest possible number of ancestral 
germ-plasms is accumulated in the germ-plasm of each germ-cell, 
and thus that the small number of loops not only means an increase 
in quantity but a multiplication in the number of different ancestral 
germ-plasms present in each of them. If this conclusion be correct, 
there can be no doubt that the second division of the egg-nucleus 
means a reduction in the above-mentioned sense. 

But there are yet other observations which, if con-ect, must also 
be considered as ' reducing divisions.' I refer to all those cases in 
which the longitudinal splitting of the loops is either entirely 
wanting, or does not occur until after the loops have left the equator 
of the spindle and have moved towards the poles. In both instances 
the bearing upon the question would be the same, for only half the 
number of primary loops would reach each pole in either case. If 
therefore the primary loops are not made up of identical pairs, it 
follows that the two daughter-nuclei can only contain half the 



THEIE SIGNIFICANCE IN HEEEDITY. 363 

number of ancestral germ-plasms which were contained in the 
mother-nucleus. Whether the loops divide on their way to the 
poles or at the poles themselves, no difference will be brought about 
in the number of ancestral germ-plasms which they contain, for 
this number can neither increase nor diminish. . The quantity of 
the different ancestral germ-plasms can alone be increased in this 
way. I am here referring to observations made by Carnoy^ on 
the cells which form the spermatozoa in various Arthropods. It 
must be admitted, however, that these divisions cannot be regarded 
as ' reducing divisions,' if Flemming's ^ suggestion be confirmed, 
that in all these observations the fact has been overlooked that the 
equatorial loops are not primary but secondary, and that they have 
arisen from the longitudinal splitting of the nuclear thread during 
previous stages of nuclear division. But this point can only be 
decided by renewed investigation. Although many excellent re- 
sults have been obtained in the subject of karyokinesis, there is still 
very much to be learnt before our knowledge is complete ; and this 
is not to be wondered at when we remember the great difiiculties in 
the way of observation which are chiefly raised by the minute size 
of the objects to be investigated. Flemming's most recent publica- 
tions prove that we are still in the midst of investigation, and that 
highly interesting and important processes have hitherto escaped 
attention. A secure basis of facts is only very gradually obtained, 
and there are still many conflicting opinions upon the details of this 
process. I should therefore consider it to be entii'ely useless, from my 
point of view, to enter into a critical examination of everything 
known about all the details of karyokinesis. I am quite content to 
have shown how it may be imagined that the reduction required by 
my theory takes place during nuclear division ; and at the same 
time to have pointed out that there are already observations which 
may be interpreted in this sense. But even if I am mistaken in 
this interpretation, the theoretical necessity for a reduction in the 
number of ancestral germ-plasms, a reduction repeated in every 
generation, seems to me to be so securely founded that the processes 
by which it is effected must take place, even if they are not supplied 
by the facts already ascertained. There must be two kinds of karyo- 

^ Carnoy, ' La Cytodi(^rfese chez les Arthropodes.' Louvain, Gand, Lierre, 1885. 
^ Flemming, 'Neue Beitrage zur Kenntniss der Zelle.' Ai'ch. f. mikr. Anat. 
Bd. XXIX, i88i. 



364 ON THE NUMBER OF POLAR BODIES AND 

kinesis according to the different physiological effect of the process. 
First, a karyokinesis by means of which all the ancestral germ-plasms 
are equally distributed in each of the two daughter -nuclei after 
having been divided into halves : secondly, a karyokinesis by means 
of which each daughter-nucleus receives only half the number of 
ancestral germ-plasms possessed by the mother-nucleus. The former 
may be called ' equal division,' the latter ' reducing division.' Of 
course these two processes, which differ so greatly in their effects, 
must also be characterized by morphological differences, but we 
cannot assume that the latter are necessarily visible. Just as, during 
the division of the first and second nuclear spindle in the egg of 
Ascaris meffalocepkala, karyokinesis takes, upon the whole, the same 
morphological course, although we must ascribe different physio- 
logical meanings to the two processes of division, — so it may be in 
other cases. The ' reducing division ' must be always accompanied 
by a reduction of the loops to half their original number, or by a 
transverse division of the loops (if such division ever occurs) ; 
although reduction can only occur when the loops are not made up 
of identical pairs. And it will not always be easy to decide whether 
this is the case. On the other hand, the form of karyokinesis in which 
a longitudinal splitting of the loops takes place before they separate 
to form the daughter-nuclei must always, as far as I can see, be 
considered as an ' equal division.' In the accompanying figures II 
and III, diagrams are given illustrating these two forms of karyo- 
kinesis, but I do not mean to imply that it is impossible to imagine 
any other form in which they may occur. 

In Figure II a nuclear spindle is seen at A^ and at its equatorial 
zone there are twelve primary loops. The transverse cross-lines 
and other markings on the loops indicate that they are composed 
of different ancestral germ-plasms. The loops are shaded differently 
in order to render the diagram clear. At B six of the loops are 
seen to have moved to either pole, so that the figure is a repre- 
sentation of the 'reducing division.' Figure III is a diagrammatic 
representation of 'equal division.' The six loops at the equatorial 
zone of A are shown by different cross-lining and shading to be 
composed of different ancestral germ-plasms. The loops split 
longitudinally in a direction indicated by the longitudinal line 
upon each of them. In B the halves of the loops are seen to 
have moved to the opposite poles of the spindle, so that there 



THEIE SiaNIFICANCE IN HEEEDITY. 



365 



are not only six loops at each pole, but also all the six combina- 
tions of ancestral germ-plasms. 

Perhaps some may be inclined to look upon direct nuclear 
division as a ' reducing- division,' but I believe that such a view 




^/l^ 



Figs. II, III. 

would be incorrect. It is only approximately true that the nuclear 
thread is divided into two halves of equal quantity by direct 
division, and exact equality would only happen as it were acci- 
dentally ; so that we cannot speak of a perfectly equal distribution 
of the ancestral germ-plasm in the two daughter-nuclei. But the 
' reducing division ' must obviously effect an exactly regular and 



366 ON THE NUMBER OP POLAR BODIES AND 

uniform distribution of the ancestral germ-plasms, although this does 
not imply that every ancestral germ-plasm of the mother-nucleus 
would be represented in each of the two daughter-nuclei. But 
if out of e. g. eight nuclear loops at the equatorial plane, four pass 
into one, and the other four into the other daughter-nucleus, each of 
the latter will contain an equal number of ancestral germ -plasms, 
although different ones. This is indeed part of the foundation 
of the theory, for the 'reducing division' must remove exactly 
half of the original number of ancestral germ-plasms, and pre- 
cisely the same number must be replaced at a later period by 
the sperm-nucleus. This could hardly be achieved with sufficient 
precision by direct nuclear division. 

I now come to inquire whether the expulsion of the second 
polar body is in reality, as I have already maintained, a reduction 
in the number of ancestral germ-plasms present in the nucleus of 
the egg. The view itself is sufficiently obvious, and it would 
supply an explanation of the meaning of the process which is still 
greatly wanted ; but it will nevertheless be not entirely useless to 
consider other possible theories. 

It would be quite conceivable to suppose that the youngest 
egg-cells, which multiply by division, may undergo one ' reducing 
division ' in addition to the ordinary process. Of course this 
should occur once only, for if repeated, the number of ancestral 
idioplasms in the nucleus of the germ-cell would undergo a decrease 
greater than could be afterwards compensated by the increase due 
to fertilization. Thus the number of ancestral germ-plasms would 
continually decrease in the course of generations, — a process which 
would necessarily end with their complete reduction to a single 
kind, viz. to the paternal or the maternal germ-plasm. But the 
occurrence of such a result is disproved by the facts of heredity. 
Although such an early occurrence of the ' reducing division ' 
would offer advantages in that nothing would be lost, for both 
daughter-nuclei would become eggs, instead of one of them being 
lost as a polar body, nevertheless I do not believe that it really 
occurs : weighty reasons can be alleged against it. 

Above all, the facts of parthenogenesis are against it. If the 
number of ancestral germ-plasms received from the parents were 
reduced to half in the ovary of the young animal, how then could 
parthenogenetic development ever take place? It is true that 



THEIE SIGNIFICANCE IN HEEEDITY. 367 

we cannot at once assert the impossibility of an early ' reducing 
division ' on this account, for as I have shown above, the power to 
develope parthenogenetically depends upon the quantity of germ- 
plasm contained in the mature egg ; the necessary amount might 
be produced by growth, quite independently of the number of 
different kinds of ancestral germ-plasms which form its constituents. 
The size of a heap of grains may depend upon the number ;of grains, 
and not upon the number of different kinds of grains. But in 
another respect such a supposition would lead to an unthinkable 
conclusion. In the first place, the number of ancestral germ-plasms 
in the germ- cells would be diminished by one half in each new 
generation arising by the parthenogenetic method ; thus after ten 
generations only j^^t of the original number of ancestral germ- 
plasms would be present. 

Now, it might be supposed that the ' reducing division ' of the 
young egg-cells was lost at the time when the parthenogenetic 
mode of reproduction was assumed by a species ; but this suggestion 
cannot hold, because there are certain species in which the same 
eggs can develope either sexually or parthenogenetically (e. g. the 
bee). It seems to me that such cases distinctly point to the fact 
that the reduction in the number of ancestral germ-plasms must 
take place immediately before the commencement of embryonic 
development, or, in other words, at the time of maturation of the 
egg. It is only decided at this time whether the egg of the bee 
is to develope into an embryo by the parthenogenetic or the sexual 
method ; such decision being brought about, as was shown above, 
by the fact that only one polar body is expelled in the first case, 
while two are expelled in the second. But if we are obliged to 
assume that reproduction by means of fertilization, necessarily 
implies a reduction to one half of the number of ancestral germ- 
plasms inherited from the parents, — the further conclusion is 
obvious, that the second division of the egg-nucleus and the expul- 
sion of the second polar body represent such a reduction, and that 
this second division of the egg-nucleus is unequal in the sense 
mentioned above, viz. one half of the ancestral germ-plasms re- 
mains in the egg-nucleus, the original number being subsequently 
restored by conjugation with a sperm-nucleus ; while the other 
half is expelled in the polar body and perishes. 

I may add that observations, so far as they have extended to 



368 ON THE NUMBER OF POLAR BODIES AND 

such minute processes, do indeed prove that the number of loops 
is reduced to one half. It has been already mentioned that, ac- 
cording to Camoj, such reduction occurs in Ascaris megalocephala, 
but the same author also describes the process of the formation 
of polar bodies in a large number of other Nematodes ^, and his 
descriptions show that the process occurs in such a way that the 
number of ancestral germ-plasms must be reduced by half. Some- 
times half the number of primary loops pass into the nucleus of the 
polar body, while the other half remains in the egg. In other 
cases, as in Ophiostomum mucronatum, the primary nuclear rods 
divide transversely, — a process which must produce the same eflFect. 
It is true that these observations require confirmation, and since, 
with unfavourable objects, the difficulties of observation are ex- 
tremely great, there may have been errors of detail ; but I do 
not think that there is any reason for doubting the accuracy of 
the essential point. And this essential point is the fact that the 
number of primary loops is divided into half by the formation of 
the polar body. 

But even if we could not admit that such a conclusion is securely 
founded, it cannot be doubted that the formation of the second polar 
body reduces to one half the quantity of the nucleus which would have 
become the segmentation-nucleus in the parthenogenetic develop- 
ment of the egg. This is a simple logical conclusion from the 
two following facts : first, parthenogenetic eggs expel only mie polar 
body ; secondly, there are eggs (such as those of the bee) in which 
it is absolutely certain that the same half of the nucleus — which 
is expelled as the second polar body in the egg requiring fertil- 
ization — remains in the egg when it is to develope parthenoge- 
netically, and acts as half of the segmentation-nucleus. But this 
proves that the expelled half of the nucleus must consist of true 
germ-plasm, and thus a secure foundation is laid for the assump- 
tion that the formation of the nucleus of the second polar body 
must be considered as a ' reducing division.' 

I was long ago convinced that sexual reproduction must be 
connected with a reduction in the number of ancestral germ-plasms 
to one half, and that such reduction was repeated in each genera- 
tion. When, in 1885, 1 brought forward my theory of the conutinuity 

^ Camoy, 'La Cytodierfese de I'oeuf; la v^sicule germinative et les globules 
polaires chez quelques Nematodes.' Louvain, Gand, Lierre. 1886. 



THEIR SIGNIFICANCE IN HEREDITY. 369 

of the germ-plasm, I had long before that time considered whether 
the formation and expulsion of polar bodies must not be inter- 
preted in this sense. But the two divisions of the egg-nucleus 
caused me to hesitate. The two divisions did not seem to admit 
of such an interpretation, for by it the quantity of the nucleus 
is not divided into halves, but into quarters. But a division 
of the number of ancestral germ-plasms into quarters would have 
caused, as was shown above, a continuous decrease, leading to their 
complete disappearance; and such a conclusion is contradicted by 
the facts of heredity. For this reason I was led at that time 
to oppose Strasburger's view that the expulsion of the polar bodies 
means a reduction of the quantity of nuclear substance by only 
half. My objection to Such a view was valid when I said that the 
quantity of idioplasm contained in the egg-nucleus is not, as a 
matter of fact, reduced to one half, but to one quarter, inasmuch 
as two successive divisions take place. I may add that I had also 
considered whether the two successive divisions might not possess 
an entirely different meaning, — whether one of them led to the 
removal of ovogenetic nucleoplasm, while the other resulted in a 
reduction in the number of ancestral germ-plasms. But at that 
time there were no ascertained facts which supported the suppo- 
sition of such a difference, and I did not wish to bring forward the 
idea, even as a suggestion, when there was no secure foundation 
for it. The morphological aspects of the formation of the first 
and second polar bodies are so extremely similar that such a 
supposition might have been considered as a mere efTort of the 
imagination. 

Hensen ^ also rejected the second part of the supposition that 
reduction must take place in the number of the hereditary elements 
of the egg, and that such reduction is caused by the expulsion of 
polar bodies, because he believed it to be incompatible with the 
fact, which had just been discovered, that polar bodies are formed 
by parthenogenetic eggs. He concludes with these words : ' If 
this striking fact be confirmed, the hypothesis which assumes that 
the egg must be divided into half before maturation, is refuted, 
and there only remains the rather vague explanation that a pro- 
cess of purification must precede the development of the embryo.' 

^ Hensen, 'Die Grimdlagen der Vererbung nach dem gegenwartigen Wissens- 
kreis,' Zeitschr. f. wissenschaftl. Landwirtlischaft, Berlin, 1885, p. 731. 

Bb 



370 ON THE NUMBER OF POLAR BODIES AND 

Nevertheless Hensen is the only writer who has hitherto taken 
into consideration the idea that sexual reproduction causes a 
regularly occurring ' diminution in the hereditary elements of 
the egg.' 

III. The Foregoing Considerations applied to the Male 

Germ-Cells. 

If the result of the previous considerations be correct, and if 
the number of ancestral germ-plasms contained in the nucleus of 
the egg-cell destined for fertilization must be reduced by one half, 
there can be no doubt that a similar reduction must also take 
place, at some time and by some means, in the gemi-plasms of the 
male germ-cells. This must be so if we are correct in maintaining 
that the young germ-cells of a new individual contain the same 
nuclear substance, the same germ-plasm, which was contained in the 
fertilized egg-cell from which the individual has been developed. 
The young germ-cells of the offspring must contain this substance 
if my theory of the continuity of the germ-plasm be well founded, 
for this theory supposes that, during the development of a fer- 
tilized egg, the whole quantity of germ-plasm does not pass through 
the various stages of ontogenetic development, but that a small 
part remains unchanged, and at a later period forms the germ- 
cells of the young organism, after having undergone an increase in 
quantity. According to this supposition therefore the germ-plasm 
of the parents must be found unchanged in the germ-cells of the 
offspring. #'If this theory were false, if the germ-plasm of the germ- 
cells were formed anew by the organism, perhaps from Darwin's 
* gemmules ' which pour into the germ-cells from all sides, it 
would be impossible to understand why it has not been long ago 
arranged that each germ-cell should receive only half the number 
of the ancestral gemmules present in the body of the parent. 
Hence the expulsion of the second polar body — assuming the 
validity of my interpretation — is an indirect proof of the soundness 
of the theory of the continuity of the germ-plasm, when contrasted 
with the theory of pangenesis. If furthermore, a kind of cyclical 
development of the idioplasm took place, as supposed by Stras- 
burger, and if its final ontogenetic stage resulted in the re-appear- 
ance of the initial condition of the germ-plasm, we should fail to 



THEIR SIGNIFICANCE IN HEREDITY. 371 

understand how any of the ancestral germ-plasms could be lost 
dm-ing- such a course of development. 

"Whichever view, the latter or the theory of the continuity of 
the germ-plasm, be correct, in either case the male germ-cells of 
the young animal must contain the same germ-plasm as that 
which existed in the fertilized maternal egg, that is to say, they 
must contain all the ancestral germ- plasms of the &ther and the 
mother. Here therefore a reduction must occur, for otherwise the 
number of ancestral germ-plasms would be increased by one half at 
every fertilization. The egg-cell would furnish |, but the sperm- 
cell f of the total quantity of germ-plasm present in the germ- 
cells of the parents. But there is no reason for believing that 
the reduction of germ-plasm in the sperm-cell must proceed in 
precisely the same way as in the egg-cell, viz. by the expulsion 
of a polar body. On the contrary, the processes of spermato- 
genesis are so remarkably different from those of ovogenesis that 
we may expect to find that reduction is also brought about in a 
different manner. 

The egg-cell does not expel the superfluous ancestral germ- 
plasms until the end of its development, and in a form which 
induces the destruction of the separated portion. This is certainly 
remarkable, for germ-plasm is a most important substance, and 
although it seems to be wasted in the production of enormous 
quantities of sperm- and egg-cells, such waste is only apparent, 
and is in reality the means which renders the species capable of 
existence. It may perhaps be possible to prove that in this case 
also the waste is only apparent. Such proof would bo forthcoming 
if it could be shown that the means by which reduction is brought 
about in eggs is advantageous, and therefore also, ceteris jparibu-9, 
necessary. We see that everywhere, as far as our observation ex- 
tends, the useful is also the actual, unless indeed it is impossible 
of attainment or can only be attained by the aid of processes which 
are injurious to the species. And if it be asked why germ-plasm 
is wasted in the maturation of egg-cells, the following may per- 
haps be a satisfactory answer. 

Let us suppose that the necessary reduction of the germ-plasm 
does not take place by the separation of the second polar body, but 
that it happens during the first division of the first primitive-germ- 
cell which is found in the embryo, so that the two first egg-cells 

B b 2 



372 ON THE NUMBER OF POLAR BODIES AND 

resulting from this division would already contain only half the 
number of ancestral germ-plasms from the father and the mother, 
contained in the fertilized egg-cell. In this case the main object, 
the reduction of the ancestral germ-plasms, would be gained by a 
single division, and all the succeeding nuclear divisions, causing the 
multiplication of these two first germ-cells, might take place by 
the ordinary form of nuclear division, viz. 'equal division.' But 
perhaps nature not only cares for this one main object alone, but 
also secures cei-tain secondary advantages at the same time. In the 
case which we have supposed the egg-cells of the mature ovaiy 
would only contain two different combinations of germ-plasm, 
which we may call combinations A and B. Even if millions of 
egg-cells were formed, every one of them would contain either A 
or B, and hence (at least as far as the female pronucleus is con- 
cerned) only two kinds of individuals could arise from such eggs — 
viz. offspring A' and B'. All the offspring A' would be as similar 
to one another as identical twins, and the same would be true 
of offspring B. 

But if the looth instead of the ist embryonic germ-cell entered 
upon the ' reducing division,' a hundred cells would undergo this 
division at the same time, and thus two hundred different com- 
binations of ancestral germ-plasm would arise, and two hundred 
different kinds of germ-cells would be found in the mature ovary. 
A still greater number of different combinations of hereditary ten- 
dencies would arise if the ' reducing division ' occuiTed still later ; 
but undoubtedly the diversity in the composition of the germ- 
plasm must be greatest of all when the ' reducing division ' does 
not take place during the period in which the germ-cells undergo 
multiplication, but at the end of the entire course of ovarian 
development, and separately in each full-grown mature egg ready 
for embryonic development. In such a case there will be as many 
different combinations of ancestral germ-plasms as there are eggs, 
for, as I have shown above, it is hardly conceivable that such a 
complex body as the nuclear substance of the egg-cell — composed 
of innumerable different units — would ever divide twice in pre- 
cisely the same manner. Every egg will therefore contain a 
somewhat different combination of hereditary tendencies, and thus 
the offspring which arise from the different germ-cells of the same 
mother can never be identical. Hence by the late occurrence of the 



THEIR SIGNIFICANCE IN HEREDITY. 373 

' reducing division ' the greatest possible variability in the offspring 
is secured. 

If my interjjretation of the second polar body be accepted, it 
is obvious that the late occurrence of the ' reducing division ' is 
proved. At the same time we receive an explanation of the ad- 
vantage gained by the postponement of the reduction of the germ- 
plasm until the end of the ovarian development of the egg ; 
because the greatest possible number of individual variations in 
the offspring are produced in this way. 

If I am not mistaken, this argument lends additional support to 
the idea which I have previously propounded, — that the most 
important duty of sexual reproduction is to preserve and con- 
tinually call forth individual variability, the foundation upon which 
the transformation of species is built ^ . 

But if it be asked whether the postponement of the 'reducing 
division ' to the end of the ovarian development of the egg is incon- 
sistent with the preservation of the other half of the dividing nucleus, 
I should be inclined to reply that a ' reducing division ' of the 
mature egg, resulting in the production of two eggs, was probably 
the phyletic precursor of the present condition. I imagine that the 
division of the mature egg-cell — although it is now so extremely 
unequal — was equal in very remote times ; but that for reasons of 
utility, connected with the specialization of the eggs of animals, it 
gradually became more and more unequal. It is now hardly pos- 
sible to give in detail the various reasons of utility which have 
brought about this condition, but it may be assumed that the 
enormous size attained by many animal egg-cells has been espe- 
cially potent in producing the change. 

A careful consideration of this last point seems to me to be 
demanded by a comparison of the egg-cells with the male germ- 
cells. Just as the female germ-cells of animals are distinguished 
by the attainment of a large size, the male germ-cells are generally 
remarkable for their minute proportions. In most cases it would 
be physiologically impossible for a large egg-cell, rich in yolk, to 
attain double its specific size in order to undergo division into two 
equal halves and yet to remain of the characteristic size. Even 
without the additional difficulties imposed by the necessity for such 

^ See the preceding Essay on 'The Significance of Sexual Beproduction in the 
theory of Natural Selection.' 



374 ON THE NUMBER OF POLAR BODIES AND 

a division, all means — such as cells used as food, or the passage of 
food from follicular cells into the ovum, etc. — are employed in 
order to bring" the egg-cell to the greatest attainable size. Fur- 
thermore, the ' reducing division ' of the nucleus cannot take place 
before the egg has attained its full size, because the ovogenetic 
nucleoplasm still controls the egg-cell, and must be removed before 
the germ-plasm can regulate its development. By arguments such 
as these I should attempt to render the whole subject intelligible. 

But the case is entirely different with the sperm-cells, which 
are generally minute: here it is quite conceivable that a 're- 
ducing division ' of the nuclei may take place by an equal division 
of the sperm-cells, occurring towards the end of the period of their 
formation ; that is to say, in such a way that both products of 
division remain sperm-cells, and neither of them perishes like the 
polar bodies. But the other possibility also demands consideration, 
viz. that the reducing division may occur at an earlier stage in the 
development of sperm-cells. At all events, the arguments adduced 
above, which proved that the consequence would be a want of vari- 
ability in the egg-cells, would not apply to an equal extent in the 
case of the male germ-cells. Among the egg-cells it may be very 
important that each one should have its special individual cha- 
racter, produced by a somewhat different composition of its germ- 
plasm, inasmuch as a considerable proportion of the eggs frequently 
developes, although this is never the case with all of them. But 
the production of sperm-cells is in most animals so enormous that 
only a very small percentage can be used for fertilization. If, 
therefore, e. g. ten or a hundred spermatozoa contained germ-plasm 
with exactly the same composition, so that, as far as the paternal 
influence is concerned, ten or a hundred identical individuals would 
result if they were all used in fertilization, such an arrangement 
would be practically harmless, for only one spermatozoon out of an 
immense number would be employed for this purpose. From this 
point of view we might expect that the ' reducing division ' of the 
sperm-nucleus would not take place at the end of the development 
of the sperm-cell, but at some earlier period. There is no necessary 
reason for the assumption that this division must take place at the 
end of development, and without some cause natural selection can- 
not operate. It is, of course, conceivable that the causes of other 
events may also involve the occurrence of this division at the end 



THEIR SIGNIFICANCE IN HEREDITY. 375 

of development ; but we do not at present know of any such causes. 
I should not consider the influence of the specific histogenetic 
nucleoplasm, i.e. the spermafcogenetic nucleoplasm, to be such a 
cause, because the quantitative proportions are very different from 
those which obtain in the formation of egg-cells, and because it is 
not inconceivable that the small quantity of true germ-plasm 
which must be present in the nuclei of the sperm-cells at every 
stage in their formation might enter upon a 'reducing division' 
with the spermatogenetic nucleoplasm, even when the latter pre- 
ponderated. 

As soon as we can recognize with certainty the forms of nuclear 
division which are ' reducing divisions,' the question will be settled 
as far as spermatogenesis is concerned. It has been already estab- 
lished that various forms of nuclear division occur at different 
periods of spermatogenesis. I make this assertion, not only from 
my own observations, but also from observations which have been 
made and insisted upon by others. Thus, van Beneden and Julin ^ 
stated in 1884 that direct and karyokinetic nuclear divisions 
alternate with each other in the spermatogenesis of Ascaris megalo- 
cephala. Again, Carnoy^ distinctly states that the different cell- 
generations in the same testis may not uncommonly exhibit con- 
siderable differences as regards karyokinesis. ' This may go so far 
that direct and indirect division may proceed simultaneously.' 
Platner^, in his excellent paper on karyokinesis in Lepidoptera, 
also points out that the karyokinesis of the spermatocytes is 
essentially different from that of the spermatogonia. According 
to his description, the latter form may be very well interpreted as 
a ' reducing division,' for no equatorial plate is formed, and the 
chromatin rods (or granules, as they are better called in this case) 
remain from the first on both sides of the equatorial plane, and 
finally unite at the opposite poles to form the two daughter-nuclei. 
Furthermore, if Camoy has correctly observed, the form of karyo- 
kinesis which I have previously interpreted as a ' reducing division ' 
occurs in the sperm-mother-cells — a karyokinesis in which the 

* E. van Beneden and Julin, ' La Spermatog^nfese chez I'Ascaride m^galoc^phale.' 

Brussels, 1884. 

^ Camoy, ' La Cytodi^r^se chez les Arthropodes.' 

^ Gustav Plainer, ' Die Karyokinese bei den Lepidopteren als Grundlage fur eine 
Theorie der Zelltheilung.' Internation. Monatsschrift f. Anatomic und Histologic, 
Bd. III. Heft 10. Leipzig, 1886. 



376 ON THE NUMBER OF POLAR BODIES AND 

chromatin rods either do not divide longitudinally, or else divide 
in this way after they have left the equatorial plate and are 
proceeding" towards the poles. Carnoy does not himself attach any 
special importance to these observations, for he only considers 
them as proofs that the longitudinal splitting of the loops may 
occur at various periods in different species — either at the equator, 
or on the way towards the poles, or even at the poles themselves. 
"We cannot conclude from the author's statements whether this 
form of nuclear division only occurs in a single cell-generation 
during spermatogenesis, as it must do if it really represents 
a ' reducing division.' Until this point is settled, we cannot 
decide with certainty whether the described form of karyokinesis 
is to be considered as the 'reducing division' for which we are 
seeking. Fresh investigations, undertaken from these points of 
view, are necessary in order to settle the question. It would be 
useless to seek fm'ther support for the theory by going into further 
details, and by critically examining the numerous observations 
upon spermatogenesis which have now been recorded. 

I will only mention that among the various nuclei and other 
bodies in different animals which have been considered by different 
observers as the polar bodies of the sperm-cells, or the cells which 
form the latter — in my opinion the paranucleus (' Nebenkern ') of 
the ' spermatides ' described by La Valette St. George ^ has the 
highest claim to be considered as the homologue of a polar body. 
But I am inclined to identify it with the first rather than the 
second polar body of the egg-cells, and to regard it as the histo- 
genetic part of the nucleoplasm which has been expelled or 
rendered powerless by internal transformations. There are two 
reasons which lead me to this conclusion : first, as I have tried to 
show above, it is probable that the ancestral germ-plasms are not 
removed by expulsion, but by means of equal cell-division ; 
secondly, my theory asserts that the histogenetic nucleoplasm 
cannot be rendered powerless until the close of histological differ- 
entiation. 

The whole question of the details of the transformations under- 
gone by the nucleus of the male germ-cells is not ready for the 

^ La Valette St. George, ' Ueber die Genese der Samenkorper.' Fiinfte Mittheilung. 
Die Spermatogenese bei den Saugethieren und dem Menschen,' Archiv f. mikrosk. 
Anat. Bd. XV. 1878. 



THEIR SIGNIFICANCE IN HEREDITY. 377 

expression of a mature opinion. From the very numerous and 
mostly minute and careful observations which have been hitherto 
recorded, we cannot conclude with any degree of certainty when 
and how the ' reducing* division ' of the nucleus takes place, nor can 
we decide upon the processes which sig-nify the purification of the 
germ-plasm from the merely histogenetic part of the nucleoplasm. 
But perhaps it has not been without value as regards future in- 
vestigation that I have tried to apply to the male germ-cells the 
views gained from our more certain knowledge of the corresponding 
structures in the female, and thus to indicate the problems which 
now chiefly demand solution. 

IV. The Foregoing Considerations applied to Plants. 

It remains to briefly consider the case of plants. Obviously, the 
'reducing division' of the germ-nuclei, if it takes place at all, 
cannot be restricted to the germ-cells of animals. There must 
be a con-esponding process in plants, for sexual reproduction is 
essentially the same in both kingdoms ; and if fertilization must 
be preceded by the expulsion of half the number of ancestral germ- 
plasms from the eggs of animals, the same necessity must hold 
in the case of plants. 

Eut whether the process always takes place in the form of polar 
bodies, and not perhaps principally, or at any rate frequently, in 
the form of equal cell-division, is another question. It is true that 
polar bodies occur in numerous plants, as we chiefly know from 
Strasburger's researches^. Strasburger shows that cells are se- 
parated by division from the germ-cells, and perish. But it seems 
to me doubtful whether we must always regard their formation as 
the removal of half the number of ancestral germ-plasms rather 
than the histogenetic nucleoplasm of the germ-cell. It appears to 
me that histogenetic nucleoplasm must be present in the highly 
differentiated vegetable germ-cells, especially in the male cells, and 
also that it must be removed during the maturation of the cell, if 
my idea of the histogenetic nucleoplasm be accepted. It is very 
possible, as I have already mentioned, that there may be quite 
indifferent germ-cells, viz. cells which are entirely without specific 
histological structure, and in such cases histogenetic nucleoplasm 

' 1. c, p. 92. 



378 ON THE NUMBER OF POLAR BODIES AND 

would be absent ; and during the maturation of such germ -cells 
no polar body would be formed for its removal. This view accords 
with the fact that polar bodies are absent in many plants. Further- 
more, I am far from maintaining that in the cases where polar 
bodies occur, they must have the above-mentioned significance. 
I only wish to point out that the reduction assumed to be neces- 
sary for the nucleus of the vegetable germ-cells is not necessarily 
to be sought for at the close of their maturation, but perhaps even 
more frequently in an equal division of the germ-cells during some 
period of their development. 

It also seems to me to be not impossible that a number of these 
vegetative ' polar bodies ' may have an entirely different signifi- 
cance, viz. to perform some special function accessory to fertiliza- 
tion, as in the so-called * ventral canal-cells ' of the higher crypto- 
gams and conifers. As we know that even the two polar bodies 
of the animal egg are not identical — although externally they are 
extremely similar, and although they arise in a precisely similar 
manner — I am even more inclined than before to consider that 
the very various ' polar bodies ' of plants possess very different 
meanings. 

But I do not feel justified in criticizing in detail the results of 
botanical investigation. I must leave the decision of such ques- 
tions to botanists, and I only desire to state distinctly that a ' re- 
ducing division ' of the nuclei of germ-cells must occur in plants 
as well as in animals. 



V. Conclusions with regard to Heredity. 

The ideas developed in the preceding paragraphs lead to remark- 
able conclusions with regard to the theory of heredity, — conclusions 
which do not harmonize with the ideas on this subject which have 
been hitherto received. For if every egg expels half the number of 
its ancestral germ-plasms during maturation, the germ-cells of the 
same mother cannot contain the same hereditary tendencies, unless 
of course we make the supposition that corresponding ancestral 
germ-plasms are retained by all eggs — a supposition which can- 
not be sustained. For when we consider how numerous are the 
ancestral germ-plasms which must be contained in each nucleus, 
and further how improbable it is that they are arranged in 



THEIR SIGNIFICANCE IN HEREDITY. 379 

precisely the same manner in all germ-cells, and finally how in- 
credible it is that the nuclear thread should always be divided in 
exactly the same place to form corresponding" loops or rods, — we are 
driven to the conclusion that it is quite impossible for the ' re- 
ducing division ' of the nucleus to take place in an identical manner 
in all the germ-cells of a single ovary, so that the same ancestral 
germ-plasms would always be removed in the polar bodies. But if 
one group of ancestral germ-plasms is expelled from one egg, and a 
diiferent group from another egg, it follows that no two eggs can 
be exactly alike as regards their contained hereditary tendencies : 
they must all differ. In many cases the differences will only be 
slight, that isj when the eggs contain very similar combinations of 
ancestral germ-plasms. Under other circumstances the differences 
will be very great, viz. twhen the combinations of ancestral germ- 
plasms retained in the egg are very different. I might here mention 
various other considerations ; but this would lead me too far from my 
subject, into new theories of heredity. I hope to be able at some 
later period to develope further the theoretical ideas which are 
merely indicated in the present essay. I only wish to show that the 
consequences which follow from my theory upon the second division 
of the egg-nucleus, and the formation of the second polar body, 
are by no means opposed to the facts of heredity, and even explain 
them better than has hitherto been possible. 

The fact that the children of the same parents are never entirely 
identical could hitherto only be rendered intelligible by the vague 
suggestion that the hereditary tendencies of the grandfather pre- 
dominate in one, and those of the grandmother in another, while 
the tendencies of the great-grandfather predominate in a third, 
and so on. Any further explanation as to why this should happen 
was entirely wanting. Others even looked for an explanation 
to the different influences of nutrition, to which it is perfectly 
true that the egg is subjected in the ovary during its later de- 
velopment, according to its position and immediate surroundings. 
I had myself referred to these influences as a partial explanation ^, 
before I recognized clearly how extremely feeble and powerless are 
the influences of nourishment, as compared with hereditary ten- 
dencies. According to my theory, the differences between the 

^ Weismann, ' Studien zur Descendenztheorie,' ii. p. 306, Leipzig, 1876, translated 
by Meldola ; see ' Studies in the Theory of Descent,' p. 680. 



380 ON THE NUMBER OF POLAR BODIES AND 

children of the same parents become intelligible in a simple manner 
from the fact that each maternal germ-cell (I shall speak of 'the 
paternal germ-cells later on) contains a peculiar combination of 
ancestral germ-plasms, and thus also a peculiar combination of 
hereditary tendencies. These latter bj their co-operation also pro- 
duce a different result in each case, viz. the offspring, which are 
characterized by more or less pronounced individual peculiarities. 

But the theory which explains individual differences by referring 
to the inequality of germ-cells, may be proved with a high degree 
of probability by an appeal to facts of an opposite kind, viz. by 
showing that identity between offspring only occurs when they have 
arisen from the same egg-cell. It is well known that occasionally 
some of the children of the same parents appear to be almost exactly 
alike, but such children are without exception twins, and there is 
every reason to believe that they have been derived from the same 
egg. In other words, the two children are exactly alike because 
they have arisen from the same egg-cell, which could of course only 
contain a single combination of ancestral germ-plasms, and there- 
fore of hereditary tendencies^. The factors which by their co- 

[} The similar concluBion that identical ova lead to the appearance of identical 
individuals was drawn from the same data by Francis Galton in 1875. See 'The 
history of the Twins, as a criterion of the relative powers of Nature and Nurture,' 
by Francis Galton, F.R.S., Journal of the Anthropological Institute, 1875, p. 391 ; 
also by the same author, ' Short Notes on Heredity, etc. in Twins,' in the same 
Journal, 1875, p. 325. 

The author investigated about eighty cases of close similarity between twins, and 
was able to obtain instructive details in thirty-five of these. Of the latter there were 
no less than seven cases ' in which both twins suffered from some special ailment or 
had some exceptional peculiarity ; ' in nine cases it appeared that ' both twins are 
apt to sicken at the same time ; ' in eleven cases there was evidence for a remarkable 
association of ideas ; in sixteen cases the tastes and dispositions were described as 
closely similar. These points of identity are given in addition to the more super- 
ficial indications presented by the failure of strangers or even parents to distinguish 
between the twins. A very interesting part of the investigation was concerned with 
the after-lives of the thirty-five twins. ' In some cases the resemblance of body and 
mind had continued unaltered up to old age, notwithstanding very difierent con- 
ditions of life,' in the other cases ' the parents ascribed such dissimilarity as there 
was, wholly, or almost wholly, to some form of illness.' 

The conclusions of the author are as follows : ' Twins who closely resembled each 
other in childhood and early youth, and were reared under not very dissimilar con- 
ditions, either grow unlike through the development of natural characteristics which 
had lain dormant at first, or else they continue their lives, keeping time like two 
watches, hardly to be thrown out of accord except by some physical jar. Nature is 
far stronger than nurture within the limited range that I have been careful to assign 



THEIR SIGNIFICANCE IN HEREDITY. 381 

operation controlled the construction of the organism were the 
same, and consequently the results were also the same. Twins 
derived from a single egg are identical : this is a statement which, 
although not mathematically proved, may be looked upon as nearly 
certain. But there are also twins which do not possess this high 
degree of similarity, and these are even far commoner than the 
others. The explanation is to be found in the fact that the latter 
were derived from two egg-cells which were fertilized at the same 
time. In most cases, indeed, each twin is enclosed in its own 
embryonic membranes, while much less frequently both twins are 
enclosed in the same membranes. In one point only the proof is 
incomplete ; for it has not yet been shown that identical twins are 
always derived from a single egg, since such an origin, together with 
a high degree of similarity, could only be established as occurring 
together in a small proportion of the cases. We therefore see that 
under conditions of nutriment which ai'e as identical as possible, 
two egg-cells develope into unlike twins, one into identical twi ns ; 
although we cannot yet affirm that the latter result invariably 
follows. It is conceivable that the stimulus for the production of 
two eggs from one may be afforded by the entrance of two sper- 
matozoa, but these latter, as was shown above, could hardly contain 
identical hereditary tendencies, and thus two identical twins would 
not arise. It appears indeed that some cases have been observed 

to the latter.' And again, ' where the maladies of twins are continually alike, the 
clocks of their two lives move regularly on, and at the same i-ate, governed by their 
internal mechanism. Necessitarians may derive new arguments from the life 
histories of twins.' 

The above facts and conclusions held for twins of the same sex, of which at 
any rate the majority are shown by Kleinwachter's observations to have been 
enclosed in the same embryonic membranes, and therefore presumably to have 
been derived from a single ovum ; but in rarer cases the twins, although also 
invariably of the same sex, were marked by remarkable differences, greater than 
those which usually distinguish children of the same family. Mr. Galton met with 
twenty of these cases. In such twins the conditions of training, etc. had been 
as similar as possible, so that the evidence of the power of nature over nurture is 
strongly confirmed. Mr. Galton writes, ' I have not a single case in which my cor- 
respondents speak of originally dissimilar characters having become assimilated 
through identity of nurture. The impression that all this evidence leaves on the 
mind is one of wonder whether nurture can do anything at all, beyond giving in- 
struction and professional training.' 

The fact that twins produced from a single ovum seem to be invariably of the 
same sex is in itself extremely interesting, for it proves that the sex of the individual 
is predetermined in the fertilized ovum. — E. B. P.] 



382 ON THE NUMBER OF POLAR BODIES AND 

in which differences have been exhibited by twins which were en- 
closed in the same embryonic membranes ; but nevertheless I be- 
lieve that two spermatozoa are not necessary to cause the formation 
of twins by a single egg. We know, it is true, from the investi- 
gations of Fol ^, that multiple impregnation produces the simul- 
taneous beginning of several embryos in the eggs of star-fishes. 
But several embryos and young animals are not developed in this 
way, for embryonic development soon ceases, and the egg dies. 

The recent observations of Bom ^ upon the eggs of the frog also 
make it very probable that a double development is produced by 
the entrance of two spermatozoa into the egg, but here also only 
monstrosities, and not twins, were produced. On the other hand, it 
has been shown that in birds twins may be produced from the same 
egg, and there is no reason for the belief that their production is 
due to multiple impregnation. But if it may be assumed that human 
twins, when identical, have been derived from a single egg, it seems 
to me to be extremely probable that fertilization was also effected by 
a single sperm-cell. We cannot understand how such a high degree 
of similarity could have been produced if two sperm-cells had been 
made use of, for we are compelled to assume that two such cells 
would very rarely contain identical germ-plasms. 

It is most probable that the egg-nucleus coalesces with the 
nucleus of a single spermatozoon, but the resulting segmentation- 
nucleus divides together with the cell-body itself, without the 
occurrence of those ontogenetic changes in the germ-plasm which 
normally take place. The nucleoplasm of the two daughter-cells 
still remains in the condition of germ-plasm, and its ontogenetic 
transformation begins afterwards — a transformation which must of 
course proceed in the same way in both cells, and must lead to the 
production of identical offspring. This is at least a possible ex- 
planation which we may retain until it has been either confirmed 
or disproved by fresh observations, — an explanation which is more- 
over supported by the well-known process of budding in the eggs 
of lower animals. 

^ Fol , ' Recherches sur la f^condation et le commencement de ITi^nc^enie.' Genfeve, 
Bale, Lyon. 1879. 

* Born, ' Ueber Doppelbildungen beim FroBch und deren Entstehung.' Breslauer 
arztl. ZeitBchrift, 1882. 



THEIR SIGNIFICANCE IN HEREDITY. 883 

VI. Recapitulation. 

To bring- tog-ether shortly the results of this essay: — the funda- 
mental fact upon which everything else is founded is the fact that 
two polar bodies are expelled, as a preparation for embryonic develop- 
ment, from all animal eggs which require fertilization, while only 
one such body is expelled from all parthenogenetic eggs. 

This fact in the first place refutes every purely morphological 
explanation of the process. If it were physiologically valueless, 
such a phyletic reminiscence of the two successive divisions of the 
egg-nucleus must have been also retained by the parthenogenetic 

egg". 

In my opinion the expulsion of the first polar body implies the 
removal of ovogenetic nucleoplasm when it has become superfluous 
after the maturation of the e^^ has been completed. The expulsion 
of the second polar body can only mean the removal of part of the 
germ-plasm itself, a removal by which the number of ancestral 
germ-plasms is reduced to one half. This reduction must also 
take place in the male germ-cells, although we are not able to 
associate it confidently with any of the histological processes of 
spermatogenesis which have been hitherto observed. 

Parthenogenesis takes place when the whole of the ancestral 
germ-plasms, inherited from the parents, are retained in the nucleus 
of the egg-cell. Development by fertilization makes it necessary 
that half the number of these ancestral germ-plasms must be first 
expelled from the q^^, the original quantity being again restored by 
the addition of the sperm-nucleus to the remaining half. 

In both cases the beginning of embryogenesis depends upon the 
presence of a certain, and in both cases equal, quantity of germ- 
plasm. This cerlain quantity is produced by the addition of the 
sperm-nucleus to the ^^^ requiring fertilization, and the beginning- 
of embryogenesis immediately follows fertilization. The partheno- 
genetic Q^^ contains within itself the necessary quantity of germ- 
plasm, and the latter enters upon active development as soon as the 
single polar body has removed the ovogenetic nucleoplasm. The 
question which I have raised on a previous occasion — ' When is the 
parthenogenetic Qg^ capable of development?' — now admits of 
the precise answer — ' Immediately after the expulsion of the polar 
body.' 



384 ON THE NUMBER OF POLAR BODIES, ETC. 

From the preceding facts and considerations the important con- 
clusion results that the germ-cells of any individual do not contain 
the same hereditary tendencies, but are all different, in that no two 
of them contain exactly the same combinations of hereditary ten- 
dencies. On this fact the well-known differences between the 
children of the same parents depend. 

But the deeper meaning of this arrangement must doubtless be 
sought for in the individual variability which is thus continuously 
kept up and is always being forced into new combinations. Thus 
sexual reproduction is to be explained as an arrangement which 
ensures an ever- varying supply of individual differences. 



VII. 

ON THE SUPPOSED BOTANICAL PEOOFS 

OF THE 

TEANSIISSION OE ACQUIRED CHAEACTEES. 

1888. 



From ' Biologisches Centralblatt,' Bd. VIII. Nr. 3 and 4, pages 65 
and 97 : April 1888. 



C C 



VII. 



ON THE SUPPOSED BOTANICAL PROOFS 

OF THE 

TRANSMISSION OF ACQUIRED CHARACTERS. 

In a lecture on heredity, delivered in 1883^,.! first brought 
forward the opinion that acquired characters cannot be transmitted; 
and I then stated that there are no proofs of such transmission, 
that its occurrence is theoretically improbable, and that we must 
attempt to explain the transformatio of species without its aid. 
Since that time many biologists have expressed their opinions 
upon the subject, some of them agreeing with me, while others 
have taken the opposite side. It is unnecessary to allude to those 
who have attacked my opinions without first understanding the 
real point in dispute, which turns upon the true meaning of the 
phrase ' acquired character,' I think it is now generally admitted 
that a very important problem is involved in this question, the 
solution of which will contribute in a decisive manner towards 
the formation of ideas as to the causes which have produced the 
transformation of species. For if acquired characters cannot be 
transmitted, the Lamarckian theory completely collapses, and we 
must entirely abandon the principle by which alone Lamarck 
sought to explain the transformation of species, — a principle of 
which the application has been greatly restricted by Darwin in 
the discovery of natural selection, but which was still to a large 
extent retained by him. Even the apparently powerful factors 
in transformation — the use and disuse of organs, the results of 
practice or neglect — cannot now be regarded as possessing any 
direct transforming influence upon a species. And the same is 
true of all the other direct influences, such as nutrition, light, 

* See the second Essay ' On Heredity.' 
C C 2 



388 ON THE SUPPOSED BOTANICAL PROOFS OF THE 

moisture, and that combination of different influences which we 
call climate. All these, with use and disuse, may perhaps produce 
great effects upon the body {soma) of the individual, but cannot 
produce any effect in the transformation of the species, simply 
because they can never reach the germ-cells from which the 
succeeding generation arises. But if — as it seems to me — the 
facts of the case compel us to reject the assumption of the trans- 
mission of acquired characters, there only remains one principle 
by which we can explain the transformation of species — the 
direct alteration of the germ-plasm, however we may imagine that 
such alterations have been produced and combined to form useful 
modifications of the body. 

The difficulty of understanding these processes of transformation 
is by no means lightened by abandoning the Lamarckian theory. 
The difficulty in fact becomes much greater, for we are now com- 
pelled to seek a different explanation of many phenomena which 
were previously believed to be understood. But this can hardly be 
regarded as a reason for not accepting the view : for we are in 
want of a correct explanation rather than one which is easy and 
convenient. We seek truth, and when we recognize that our 
path is leading in a wrong direction, we must leave it and take 
another road even if it presents more difficulties. 

My theory rests, on the one hand, upon certain theoretical con- 
siderations which will be mentioned below, and which I have 
attempted to develope in previous papers ^. On the other hand, it 
rests upon the want of any actual proof of the transmission of acquired 
characters. My theory might be disproved in two ways, — either 
by actually proving that acquii'ed characters are transmitted, or by 
showing that certain classes of phenomena admit of absolutely 
no explanation unless such characters can be transmitted. It 
will be admitted, however, that we must be very cautious in 
accepting proofs of this latter kind, for the impossibility of ex- 
plaining a given phenomenon may be merely temporary, and may 
disappear with the progress of science. No one could have explained 
the useful adaptations so common in animals and plants, before the 

' Consult 'Ueber die Vererbung,' Jena, 1883; 'Die Kontinuitiit dea Keim- 
plasmas,' Jena, 1885 ; ' Ueber die Zahl der Richtungskorper und tiber ilire Bedeu- 
tung fiir die Vererbung,' Jena, 1887. These papers are translated as the second, 
fourth and sixth Essays in the present volume. 



TRANSMISSION OF ACQUIRED CHARACTERS. 389 

light of the theory of natural selection had fallen on these pheno- 
mena; at that time we should have been far from right if we 
had assumed that organisms possess a power which causes them 
to respond to external influences by useful modifications, a power 
unknown elsewhere, entirely unproved and only supported by the 
fact that at that time it did not seem possible to explain the 
phenomena in any other way. 

Although my theory has not been disproved, I will nevertheless 
attempt to bring into further accordance with it certain phenomena 
which seem at first sight to oppose it. I first began to take this 
course in my paper ' On Heredity^.' In that paper I attempted to 
show how the fact that disused org'ans become rudimentary may 
be readily explained without assuming the transmission of acquired 
characters ; and also that the origin of instincts may in all cases 
be referred to the process of natural selection^, although many 
observers had followed Darwin in explaining them as inherited 
habits, — a view which becomes untenable if the habits adopted 
and practised in a single life cannot be transmitted. 

Other phenomena which appeared to present difficulties were 
also considered and brought into accordance with the theory, and 
I think that I have been successful in showing that adequate and 
simple explanations may be given. 

There certainly remain many phenomena which seem to be 
opposed to my theory and for which a new explanation must be 
found. Thus Romanes^, following Herbert Spencer*, has recently 
pointed to the phenomena of correlation as a proof of the trans- 
mission of acquired characters ; but, at no distant time, I hope to 
be able to consider this objection, and to show that the apparent 
support given to the old idea is in reality insecure and breaks 
down as soon as it is critically examined. I believe that I shall 
be able to prove that correlation cannot be used as the indirect 
proof of an hypothesis, of which all direct evidence is still com- 

* See the second Essay. 

[^ See R. Meldola in Ann. and Mag. Nat. Hist., 1878, vol. i. pp. 158-161. The 
author discusses many cases among insects in which instinct is related to protective 
structure or colouring : he also considers that instinct is to be explained by the 
principle of natural selection which a,ccounts for the other protective features. — 
E. B. P.] 

[^ See ' Nature,' vol. 36, pp. 491-407. — E. B. P.] 

[* See ' The Factors of organic Evolution ' in * The Nineteenth Century ' for April 
and May 1886.— E. B. P.] 



390 ON THE SUPPOSED BOTANICAL PROOFS OF THE 

pletely wanting. It must not be forgotten that the onus prohandi 
rests with my opponents : they defend the assertion that acquired 
characters can be transmitted, and they ought therefore to bring 
forward actual proofs; for the mere fact that the assertion has 
been hithei-to accepted as a matter of course by almost everyone, 
and has only been doubted by a very few (such as His, du Bois- 
Reymond, and Pfliiger), cannot be taken as any proof of its validity. 
Not a single fact hitherto brought forward can be accepted as 
a proof of the assumption. Such proofs ought to be found : facts 
ought to be discovered which can only be understood with the 
aid of this hypothesis. If, for instance, it could be shown that 
artificial mutilation spontaneously re-appears in the offspring with 
sufficient frequency to exclude all possibilities of chance, then such 
proof would be forthcoming. The transmission of mutilations has 
been frequently asserted, and has been even recently again brought 
forward, but all the supposed instances have broken down when 
carefully examined. I think I may here safely omit all fm-ther 
reference to the proofs dependent upon transmitted mutilations, 
especially as Doderlein^ has already, in the most convincing manner, 
disposed of the argument derived from the tailless cats which 
were so triumphantly exhibited at the last meeting of the Associa- 
tion of German Naturalists ^. 

I now come to the real subject of this paper — the supposed 
botanical proofs of the transmission of acquired changes. The 
botanist Detmer has recently brought forward certain phenomena 
in vegetable physiology ^, as a support for the transmission of such 
changes, and although I do not believe that they will bear this 
interpretation, the discussion of them may perhaps be useful, I am 
even inclined to think that these and a few other phenomena in 
vegetable physiology, upon which I shall also touch, are very likely 
to throw new light upon the whole question which has been so 
frequently misunderstood. I should have preferred to leave this 
discussion to a botanist, but I do not know whether my views will 
meet with any support from the followers of this subject, and I 
must therefore attempt the discussion myself. And perhaps it is 
of some assistance in clearing up the question, for one who is not 

* See 'Biol. Centralbl.' Bd. VII. No. 23. 
« See the next Essay (VIII). 

' Detmer, 'Zum Problem der Vererbung,' Pfluger's Archiv f. Physiologic, Bd. 41, 
(1887), p. 203. 



TKANSMISSION OF ACQUIRED CHAKAOTERS. 391 

accustomed to the usual botanical views, and is more conversant 
with other classes of biological knowledge, to consider the facts 
brought to light by modern botany, from a general point of view. 
Of course I shall not attempt to question the validity of the obser- 
vations, nor even the accuracy with which the facts have been 
interpreted. I shall only deal with the conclusions which may be 
drawn from the facts, and I do not think that it is absolutely 
necessary that such criticism should be made by a botanist. 
Questions of general biological significance such as that of heredity 
cannot be entirely solved within the single domain of either 
zoological or botanical facts. Both botanists and zoologists must 
give due weight to the facts of the province which is not their 
own, and must see whether the views which they have chiefly 
gained in the one province can be applied to the other, or whether 
phenomena occur in the latter which are in opposition to their 
previously formed views and which cause them to be abandoned 
or modified. 

Detmer begins by bringing forward certain facts which prove, as 
he believes, that rather important changes in the organism can be 
directly produced by external influences. He is of opinion that I 
under-estimate the weight of these influences, and that I make light 
of the changes which may thus arise in a single individual life. 
But obviously, it is of no importance for the question of the trans- 
mission of acquired characters, whether the changes directly pro- 
duced by external influences upon the soma of an individual are 
greater or smaller : the only question is whether they can be 
transmitted. If they can be ti'ansmitted, the smallest changes 
might be increased by summation in the course of generations, 
into characters of the highest degree of importance. It is in this 
way that Lamarck and Darwin have supposed that an organism is 
transformed by external influences. It is therefore interesting to 
see what Detmer considers to be a. change which has been directly 
efiected. We can in this way gain a very distinct appreciation of 
the difference in views which is caused by the different spheres of 
experience which belong to botany and zoology. It will be useful 
to gain a clear idea of the differences which are thus caused. 

Detmer first alludes to the dorso- ventral structure of the shoots 
of Thuja occidentalis, chiefly shown in the fact that the upper 
sides of these shoots contain the green palisade cells, while the 



392 ON THE SUPPOSED BOTANICAL PROOFS OF THE 

under sides which are turned away from the light possess green 
spheroidal (isodiametrie) cells. If the branches of Thnja are turned 
upside down and fixed in this position before the production of new 
shoots, it is found that the anatomical structure of the latter, when 
developed, is reversed. The side of the shoot which was destined 
to become the under side, but which was artificially compelled to 
become the upper side, assumes the structure of the upper side and 
developes the characteristic palisade parenchyma ; and on the other 
hand, the under side which was intended to become the upper side 
developes the spongy parenchyma which is characteristic of the 
under side. From these facts Detmer concludes that the dorso- 
ventral structure of the shoots of Thuja has resulted from the 
continual operation of an external force, and that the light must be 
considered as the cause of the structural change. 

But such a conclusion obviously depends upon a confusion of 
ideas. No one will doubt that the light was the stimulus which 
led to the reversal of the structures in the shoot, but this is a very 
difierent thing from maintaining that it was the cause which 
conferred upon the TJmja-^oo\^ the power of producing palisade 
and spongy parenchyma. When a phenomenon only occurs under 
certain conditions, it does not follow that these conditions are the 
cause of the phenomenon. A certain temperature is necessary for 
the development of a bird in the ^^^^ but surely no one will 
maintain that the temperature is the cause of the capacity for 
such development. It is obvious that the &^^ has acquired the 
power of producing a bird chiefly as the result of a long phyletic 
course of development which has led to such a chemical and physical 
structure in the ^^^ and the fertilizing sperm -cell, that after their 
union and development, a bird, and only a bird of a particular 
species, must be produced. But of course certain conditions must 
be fulfilled in order that such development may take place ; and a 
definite temperature is one of these conditions of development. 
Thus we may briefly say that the physical nature of the ^'^^ is the 
cause of its development into a bird, and we may similarly maintain 
that the physical nature of a 2"/iv(;'a-shoot, and not the influence of 
light, is the cause of the development of tissues which are character- 
istic of the species. In the development of such a shoot the light 
plays precisely the same part which is played by temperature in the 
development of a bird : it is one of the conditions of development. 



TRANSMISSION OF ACQUIRED CHARACTERS. 393 

There is nevertheless a difference between these two cases in that 
the Thtja-shoot possesses the possibility of development in two 
different ways instead of only one. The upper side of the shoot 
can assume the structure of the under side and vice versa, and this 
structural reversal depends upon the way in which the lig-ht is 
thrown upon the shoot. But even if the lig-ht causes the structural 
reversal, does this justify us in assuming* that the structure itself 
is also the direct consequence of the influence of light ? I see no 
reason for rejecting the supposition that the physical natm^e of part 
of a plant may be of such a kind that this or that stmcture may be 
produced according- as this or that condition of development pre- 
vails. Thus with stronger light the structure of the upper side of 
the shoot developes ; with weaker light, the structure of the under 
side. But this physical nature of the TAuJa-hnd depends, like that 
of a bird's egg, upon its phyletic history, as we must assume to be 
the case with, the germs producing all individual developments. 
It is therefore quite impossible to interpret the reversal of the 
structure in the T/wj'a-shoot as the result of modification produced 
by the direct influence of external conditions. It is an instance 
of double adaptation — one of those cases in which the specific 
nature of a germ, an organism, or a part of an organism, possesses 
such a constitution that it reacts differently under the incidence of 
different stimuli. 

An entirely analogous example of reversal occurs in the climbing 
shoots of the Ivy, and is described in Sachs' lectures on the 
physiology of plants. Such shoots produce leaves only on the side 
directed towards the light, and roots (which are made use of in 
climbing) only upon the opposite side. If however the position of 
the plant be altered so that the root-bearing side is turned towards 
the light, while the leafy side is shaded, a reversal occurs, so 
that from that time the former only produces leaves, and the latter 
nothing but roots. In other words, the Ivy-shoot reacts under 
strong light with the production of leaves and under weak light 
with the production of roots, just as litmus-paper becomes red with 
an acid and blue with an alkali. The physical nature of the Ivy- 
shoot was present before the production of either structure, and was 
no more due to the action of light itself, than the physical nature 
of litmus-paper is due to an acid or an alkali. But this is quite 
consistent with the possession of a physical nature which reacts 



394 ON THE SUPPOSED BOTANICAL PROOFS OF THE 

diflPerently under the two different conditions afforded by light and 
shade. 

No one would think of bringing forward the changes in the 
colour of the green frog {Ityla) as a proof of the power of direct 
influences in causing structural modifications in the animal body. 
The frog is light green when it is resting upon green leaves, but it 
becomes dark brown or nearly black when transferred to dark 
surroundings. This is an obvious instance of adaptation, for the 
changes in the colour of the frog depend upon a complex reflex 
mechanism. The changes in the shape of the chromatophores of 
the skin are not produced by the direct influence of the different 
rays of light upon the body-sui-face, but in consequence of the 
action of these rays upon the retina. Blind frogs do not react 
under the changes of light. Hence it is impossible that any one 
can maintain that the skin of the frog has gained its green colour 
as the direct result of the green light reflected from its usual sur- 
roundings. It must be admitted that in this and in all similar 
cases, there is only one possible explanation, viz. an appeal to the 
opemtion of natural selection. It may be objected that we are not 
here dealing, as in the Thuja and Ivy, with changes in the course 
of ontogenetic development following upon the occurrence of this or 
that external condition, but only with the different reactions of 
a mature organism. But nevertheless, cases of the former kind 
appear to be also present in the animal kingdom. 

Thus the very careful and extensive investigations of Poulton^ upon 
the colours of certain caterpillars have distinctly diown that some 
species possess the possibility of development in two directions, and 
that the actual direction taken by the individual is decided by the 
influence of external conditions. Poulton surrounded certain larvae 
of Geometrae with an abundance of dark branches, in addition to 
the leaves upon which they fed. When such conditions prevailed 
from the beginning of larval life, the caterpillars as they developed, 
gradually assumed the daik colour of the twigs and branches upon 
which they rested. When other larvae of the same species (and in 

[' Dr. Weismann is here alluding to experiments upon the larvae of Eumia Cra- 
iaegata. A short account of the results will be found in the Eeport of the British 
Association at Manchester (1887), and in 'Nature,' vol. 36, p. 594. I have now 
obtained similar results with many other species (see Trans. Ent. Soc., Lond. 1888, 
p. 553) ; but many of the results are as yet unpublished. — E. B. P.] 



TRANSMISSION OF ACQUIRED CHARACTERS. 395 

many experiments hatched from the same batch of eggs) were 
similarly exposed to the green leaves of the same food-plant, they 
did not indeed become bright green like the leaves, but were in- 
variably of a much lighter colour than the other larvae, while 
many of them gained a brownish-green tint. The larvae of 
Smerinthus ocellatus^ also possess the power of assuming different 
shades of gi'een and of thus approaching, to some extent, the green 
of the plant upon which they happen to live. It is quite impossible 
to explain the phyletic development of the green colour of these 
and other caterpillars as due to the direct action upon the skin of 
the green light reflected from the leaves upon which they sit. 
The impossibility of such an effect was pointed out long ago by 
Darwin, and also followed from my own investigations. Here, as 
in the other cases, the only possible solution is afforded by natural 
selection. The colour of the caterpillars has become gradually more 
and more perfectly adapted to the colour of the leaves, — and often 
to the particular side of the leaves upon which these animals rest, 
• — not by the direct effect of reflected light, but by the selection of 
those individuals which were best protected. Poulton's experi- 
ments quoted above prove that certain species which occur upon 
different plants with different colours (or even in some cases upon 
the differently coloured parts of the same plant), present us with 
a further complication in the process of adaptation, inasmuch as 
each individual has acquired the power of assuming a lighter or 
darker colom*^. The light which falls upon a single individual 

[^ See the editorial notes by Raphael Meldola, in his translation of Weismann's 
' Studies in the Theory of Descent ' (the Essay on ' The Origin of the Markings of 
Caterpillars,' pp. 241 and 306): also E. B. Poulton, in 'Proc. Roy. Soc.,' vol. 
xxxviii. pp. 296-314; and in 'Proc. Roy. Soc.,' vol. xl. p. 135. — E. B. P.] 

[^ Professor Meldola first called attention to the scattered instances of the kind 
here alluded to by Professor Wei smann, in 1873 : see ' Proc. Zool. Soc.,' 1873, P- ^SS- 
The author explains the relation of this ' variable protective colouring' to other 
protective appearances, and he is strongly of the opinion that the former as well as 
the latter is to be explained by the action of the ' survival of the fittest.' 

The validity of Dr. Weismann's interpretation of these effects as due to adapta- 
tion, through the operation of natural selection, is conclusively proved by the follow- 
ing facts. The light reflected from green leaves becomes the stimulus for the 
production of darh brown pigment in those cases in which the leaves constitute the 
surroundings for many months. Under these circumstances the leaves of course 
become brown at a relatively early date, and protection is thus afforded for the 
remainder of the period, although the dark pigment is produced before the change in 
the colour of the leaf. Instances of this kind are seen in the colours of cocoons spun 
among leaves by certain lepidopterous larvae (see * Proc. Ent. Soc. Lond.,' 1887, pp. 



896 ox THE SUPPOSED BOTANICAL PllOOFS OF THE 

caterpillar during the course of its g-rowth determines whether the 
lighter or darker colour shall be developed. Here therefore we have 
a case exactly parallel to that of the IVwj a-shoot in which the 
palisade or spongy parenchyma is developed according to the 
position in which the shoot is fixed. 

As far as it is possible in the present condition of our knowledge 
to offer any opinion upon the origin of sex in bisexual animals, it 
may be suggested that this problem is also capable of an essentially 
similar solution. Each germ-cell may possess the possibility of 
developing in either of two directions, the one Resulting in a male 
individual, and the other resulting in a female, while the decision as 
to which of the two possible alternatives is actually taken may rest 
with the external conditions. We must, however, include among the 
external circumstances everything which is not germ-plasm. More- 
over, this explanation is by no means certain, and I only mention 
it as an instance which, if we assume it to be correct, further illus- 
trates my views upon the phenomena presented by the T/mJa-shoot. 

The two other facts brought forward by Detmer as proofs of 
the transforming power of external influences can be explained 
in precisely the same manner. These instances are — the fact 
that Tropaeolum when grown in moist air produces leaves with 
anatomical characters different from those produced when the plant 
is grown in dry air ; and the differences in the structure of the 
leaves of many plants, according as they have been grown in the 
sun or shade respectively. Such differences do not by any means 
afford proof of the direct production of structural changes by means 
of external influences. How would such an explanation be con- 
sistent with the fact that the leaves are^ in all these cases, changed 
in a highly purposeful manner ? Or is it assumed that these organs 
were so constituted froni the beginning, that they are compelled 
to respond to external conditions by the production of useful 

1, li, and 1888, p. xxviii), the cocoons of the same species being of a creamy white 
colour when spun upon white paper. 

Conversely, the light reflected from the same surfaces serves as the stimulus for 
withholding figment in the cases alluded to by Dr. Weismann (larvae of J?. 
Crataegata, &c.), in all of which the organism only remains in contact with the 
leaves while they are green, viz. at a time when the dark colour would be dis- 
advantageous. 

Hence precisely opposite effects are produced by the operation of the same force ; 
the nature of the effect which actually follows in any case being solely determined 
by the advantage afforded to the organism, — E. B. P.] 



TEANSMISSION OF ACQUIRED CHARACTEES. 397 

changes ? Any one who made such an assertion nowadays, or 
who even thought of such a thing- as a possibility, would prove 
that he is entirely ignorant of the facts of organic nature, and 
that he has no claim to be heard upon the question of the 
transformation of species. The very first necessity in any scientific 
question is to gain acquaintance with that which has been thought 
and said upon the subject. And it has been frequently shown 
that whole groups of useful characters cannot by any possibility 
have been produced by the direct action of external influences. If 
a caterpillar, which hides itself by day in the crevices of the bark, 
possesses the same colour as the latter, while other caterpillars 
which rest on leaves are of a green colour, these facts cannot be 
explained as the results of the direct influence of the bark and 
leaves. And it would be even less possible to explain upon the 
same principle all the details of marking and colour by which these 
animals gain still further protection. If the upper side of the 
upper wings of certain moths is grey like the stone on which they 
rest by day, while in butterflies the under side of both wings 
which are exposed during rest, exhibits analogous protective 
colours, these facts cannot be due to the direct influence of the 
surroundings which are resembled, but, if they have arisen in any 
natural manner, they must have been indii-ectly produced by the 
surroundings. One may reasonably complain when compelled to 
repeat again and again these elements of knowledge and of thought 
upon the causes of transformation ! 

Any one who remembers these things, and is aware of the 
countless number of purposeful characters which cannot possibly 
depend upon such direct influences, will be very cautious in yielding 
to any single instance which at first sight aj)pears to be the direct 
consequence of external conditions. If" Detmer had been thus 
cautious he would hardly have written the following sentence as a 
resume of the physiological experiments on plants which have been 
already alluded to ; ' In certain cases it is possible, as we have seen, 
to artificially modify the anatomical structure of certain parts of 
plants. In such cases the relation between the structure and the 
external influences is undoubtedly clear: the latter act as the 
cause ; the anatomical structure of the members of the plant 
is the consequence of this cause.' A little more logic wovild 
have prevented the author from expressing such an opinion. 



398 ON THE SUPPOSED BOTANICAL PROOFS OP THE 

for, as has been already shown, it is founded on a confusion 
between the true cause of a phenomenon and one of the conditions 
which are necessary for its production. We might as well con- 
sider the phenomena of geotropism^ ht/drotropum^ and heliofropism — 
which have been established, and investigated in such a brilliant 
way by modern vegetable physiologists — as the direct results 
of the attraction of the earth, of water, and of light ; and it is 
not improbable that some botanists are even inclined to make this 
assumption. And yet it is perfectly easy to show that this cannot 
be the case. By geotropism we mean the power possessed by the 
parts of a plant of growing along lines which make certain 
angles with the direction of the earth's attraction. For example, 
the chief root grows parallel with the earth's attraction, viz. towards 
the centre of the earth, and it is described as positively geotropic : 
conversely the main shoot grows along the same line but in an 
opposite direction, and it is negatively geotropic. But geotropism 
is not a primitive attribute of the plant, and it is even now absent 
from those plants which, like many Algae, have no definite position. 
Geotropism cannot have arisen before plants first became fixed in 
the earth. If any one were to assume that the direct influence of 
gravity, continuous through countless generations, had at length 
conferred upon the root the power of growing in a geotropic direc- 
tion, how would it be possible to explain the fact that the shoot 
which has been under precisely the same influence has acquired the 
power of growing in an exactly opposite direction ? The chai'acter- 
istic difierences between root and shoot cannot have appeared until 
the plant became fixed in the ground, and how can we imagine that 
the same influence of gravity has since that time directly produced 
the two antagonistic results of positive and negative geotropism, 
in two structures, which were originally and essentially similar ? It 
should also be remembered that it is only the main root which 
exhibits true positive geotropism. The lateral roots form angles 
with the main root, and do not therefore grow towards the earth's 
centre ; and the same is true of the lateral shoots which grow 
obliquely, and not perpendicularly upwards, like the main shoot. 
Moreover the angles which the lateral roots make with the main 
root, and the lateral shoots with the main shoot, are quite different 
in different species. How is it possible that all these different 
modes of reaction witnessed in the different parts of plants can be 



TRANSMISSION OF ACQUIRED CHARACTERS. 399 

the direct results of one and the same external force ? It is quite 
obvious that these are all cases of adaptation. The main root has 
not acquired the power of growing* perpendicularly downwards 
under the stimulus of gravity, because this force has acted upon it 
for numberless g-enerations, but because such a direction for such a 
part was the most useful to the plant. Hence natural selection has 
conferred upon the root the power of reacting- under the stimulus of 
gravity by growing in a direction parallel to this force. For the 
main shoot, the opposite reaction was the most useful and has been 
established by natural selection, while still another reaction has been 
similarly established for the lateral roots and another for the lateral 
shoots. 

Each part of a plant has received its special mode of reacting 
under the stimulus of gravity because it was useful for the whole 
plant, inasmuch as the position of its different parts relatively to 
one another and to the soil became thus fixed and regulated. These 
modes of reaction have become different in different species, because 
the conditions of life peculiar to each require special an-angements. 

The same argument also holds with regard to heliotropism. 
The power of growing towards the light possessed by green shoots 
cannot be a primitive character of the plant : it must have arisen 
secondarily. If it were an essential and original character it could 
not be reversed in certain parts of the plant ; but the roots are 
negatively heliotropic, for they grow away from the light. There 
are also shoots, such as the climbing shoots of Ivy, which are simi- 
larly negatively heliotropic. Whenever the heliotropic power is 
thus reversed in shoots, the change is of a useful kind. Thus the 
shoots of the Ivy gain the power of clinging closely to a perpen- 
dicular wall or to some horizontal plane ^. In this case, however, 
it is only the shoot which is negatively heliotropic, its leaves turn 
towards the light ; and the same is true of the flower-bearing shoots 
which do not climb. All these are clearly adaptations and not the 
results of direct influence. The light only provides the stimulus 
which calls forth the characteristic reaction from each part of the 
plant, but the cause of each peculiar reaction lies in the specific 
nature of the part itself which has not been produced by light, but 
as we believe by processes of natural selection. If this explanation 

' Compare Sachs, 'Lectures on the Physiology of Plants,' translated by H. 
Marshall Ward, p. 710. 



400 ON THE SUPPOSED BOTANICAL PROOFS OF THE 

does not account for the facts we may as well abandbn all attempts 
at understanding- the useful arrangements in organisms. 

Sachs has used the term mmolropism to express the fact that 
the various organs of a plant assume the most diverse dii'ections of 
growth under the influence of the same forces. He also states that 
anisotropism is one of the most general characteristics of vegetable 
organization, and that it is quite impossible to form any idea as to 
how plants would appear or how they could live if their different 
organs were not anisotropic. Since anisotropism is nothing more 
than the expression of different kinds of susceptibility to the action 
of gravity, light, &c., it is obvious that the configuration of the 
plant is to be traced to such specific susceptibilities. 

Now these specific susceptibilities cannot have been produced by 
the direct effect of the various external influences (as w^as shown 
above), and the only other possible explanation is to recognise them 
as adaptations, and to admit that they have arisen by the opera- 
tion of natural selection upon the general variability of plant 
organization. 

Simple as these conclusions are, I have failed to meet with them 
in any of the writings of botanists, and they may perhaps be of use 
in helping to shake the vaguely-felt opinion that the characters of 
plants are to be chiefly referred to the direct action of external 
influences. 

At all events it cannot be maintained that the phenomena of 
anisotropism support the opinion mentioned above ; and the mere 
assertion that it is highly probable that hereditary characters 
arise as the result of external influences, is no more than the ex- 
pression of an unfounded individual opinion. It is remarkable that 
Detmer should make such an assertion as the outcome of his dis- 
cussion of the reversed Thuja-Bhoot, &c., for even if we admit that 
the dorso-ventral structure of the shoot is — as Detmer believes — 
the direct and primary effect of the action of light, the experiment 
with the reversed shoot would prove that no part of this effect 
has become hereditary. Although the upper side of the shoot 
has produced the palisade parenchyma under the influence of light 
for thousands of generations, there is nevertheless no tendency 
towards the establishment of any hereditary effect, for as soon as 
the upper side of the growing shoot is artificially transformed into 
the under side, its normal structure is at once abandoned. Hence 



TRANSMISSION OF ACQUIRED CHARACTERS. 401 

SO far from lending" any support to the assumption that acquired 
characters can be transmitted, Detmer's experiment rather tends to 
disprove this opinion. 

I think I have sufficiently shown that Detmer's reproach — that 
I have under-estimated the effects of external influences upon an 
org-anism — may be fairly directed ag-ainst its author. If we can 
believe that every structural arrangement in plants, which depends 
upon certain external conditions, has been produced in a phyletic 
sense by these latter, it becomes very easy to explain the trans- 
formation of species ; but in accepting" such an explanation we 
are building" without any foundation, for the proof that acquired 
characters can be transmitted has yet to be g-iven. 

As a further disproof of my views Detmer quotes the so-called 
phenomena of correlation in plants, and he believes that these 
instances help us to conceive how the acquired changes of the body 
(soma) of the plant may also influence the sexual cells. If the 
apical shoot of a yovmg spruce fir be cut off", one of the lateral 
shoots of the whorl next below the section rises and becomes an 
apical shoot : it not only assumes the orthotropic growth of such 
a shoot, but also its mode of branching. The phenomenon itself is 
well known, and I have often observed it myself in my garden 
without making any botanical experiments ; for this experiment 
is not uncommonly made by Nature herself, when the apical 
shoot is destroyed by insects (for example the gall-making 
Chermes). The change of the lateral into an apical shoot occurs 
here in consequence of the loss of the true apical shoot, and is 
therefore really dependent upon it. The only difficulty is to 
understand how these and many other kindred phenomena can 
be considered to prove the transmission of acquired characters. 
That correlation exists between the parts of an organism, that cor- 
related changes are not only common but nearly always accompany 
some primary change, has been perfectly well known since Darwin's 
time, and I am not aware that it has been disputed by any one. I 
further believe that hardly any one would maintain that it is im- 
possible for the reproductive organs to be influenced by correlation. 
But this is very far from the admission that such changes would 
occur in the germ-cells as would be necessary for the transmission 
of acquired characters. For such transmission to occur it would be 
necessary for the germ-plasm (the bearer of hereditary tendencies) 

Dd 



402 ON THE SUPPOSED BOTANICAL PROOFS OF THE 

to underg-o a transformation corresponding to that produced by 
the external influences ; — such a transformation as would cause the 
future organism to spontaneously develope changes similar to those 
which its parent had acquired. But since the g-erm-plasm is not 
an organism in the sense of being a microscopic facsimile which 
only has to increase in size in order to become a mature organism, 
it is obvious that the developmental tendencies must exist in the 
specific molecular structure, and perhaps also in the chemical consti- 
tution of the germ-plasm itself. It therefore follows that the changes 
in the germ-plasm which would be required for the transmission of 
an acquired character must be of an entirely different nature from 
the change itself acquired by the body of the parent plant : and 
yet it is supposed that the former is produced by the latter as a 
result of correlation. I will illustrate this by an example. Let us 
suppose that the influence of climate had caused a plant to change 
the form of its leaves from an ovate into a lobate shape : now such 
a change could not be transferred to the germ-plasm in the pollen 
and the ovules, as anything similar to leaves or the form of leaves ; 
for such specialized morphological features have no existence in the 
germ-plasm. The only thing which could happen would be changes in 
its molecular structure which bear no resemblance to those changes 
which are implied by the direct alteration of the form of the leaf 
in the parent plant. Any one who clearly appreciates this difficulty 
will hesitate in admitting the possibility of the transmission of 
acquired characters, because it is possible that the sexual cells may 
be affected by correlated influences. If the change in the form of a 
leaf exercises any influence at all upon the germ-plasm, why should 
it produce a corresponding (in the above-mentioned sense) change 
in its molecular structure ? Why should it not produce some other 
out of the immense number of possible changes ? There must be as 
many possible changes in the structure of germ-plasm as there are 
possible variations in each part of a plant that arises from it. Why 
then should the corresponding change always occur, — a change 
which had never previously existed in the whole phyletic develop- 
ment of the organic world ; foY the plant with the latest modifica- 
tion can have never existed before ? The occurrence of a particular 
change out of the countless possible changes would be about as 
likely as if one out of a hundred thousand pins thrown out of a 
window were to balance on its point when it reached the ground. 



TRANSMISSION OF ACQUIRED CHARACTERS. 403 

The assumption scarcely deserves to be called a scientific hypothesis, 
and yet it must be made by all who accept the transmission of 
acq^uired characters, — that is unless they adopt the hypothesis of 
pangenesis, which is quite as improbable, and which even Darwin 
did not look upon as a real, but only as a formal explanation. 

Detmer is also greatly mistaken when he says that I refuse to 
admit the transmission of acquired characters, because I am pre- 
judiced in favour of my doctrine of the continuity of the germ- 
plasm. This doctrine is either right or wrong, and there is no 
middle course : to this extent I quite admit that I am prejudiced. 
But the question as to whether acquired characters can be im- 
pressed upon the germ and thus transmitted would not be by any 
means settled in this way; for even if we admit that the germ- 
plasm is not continuous from one generation to another, but that 
it must be produced afresh in each individual, this would by no 
means necessarily imply that it would potentially receive and retain 
every chang-e produced in every part of the individual, and at any 
time in its life. It seems to me that the problem of the transmis-* 
sion or non-transmission of acquired characters remains, whether the 
theory of the continuity of the germ-plasm be accepted or rejected. 

I will now proceed to examine the last group of phenomena 
which Detmer brings forward in favour of the transmission of 
acquired characters. He charges me with not having taken into 
account, in discussing the problem of heredity, the very important 
facts which are known about the strange phenomena of 'after- 
effect ' in plants. Among these ' after-effects ' are the following. 

If vigorous plants of the sun-flower, grown in the open air, be 
cut off close to the ground and transferred to complete darkness, 
the examination of a tube fixed to the cut surface of the stem will 
show that the escape of sap does not take place uniformly, but 
undergoes periodical fluctuation, being strongest in the afternoon 
and weakest in the early morning. Now the cause of this daily 
periodicity in the flow of sap depends upon the periodical changes 
due to the light to which the plant was exposed when it was 
growing under normal conditions. "When plants which have been 
grown in darkness from the first are similarly treated, the flow of 
sap does not exhibit any such periodicity. 

Another instance is as follows : — it is well known that darkness 
accelerates, while light retards the growth of plants, and therefore 

D d 2 



404 ON THE SUPPOSED BOTANICAL PROOFS OF THE 

plants usually grow more strongly by night than by day. If now 
plants are transferred from the open air into constant darkness, the 
periodicity in their growth does not immediately disappear, and 
often persists for a long time as a phenomenon of after-effect. 

The opening and closing of the leaves oi Mimosa pudica also takes 
place periodically under natural conditions, the leaves closing at 
dusk as a result of changes in the stimulus provided by the light. 
In this case also, when the plants are transferred to constant dark- 
ness, the periodicity in the movements of the leaves continues for 
several days. 

All this is certainly very interesting, and it proves that periodical 
stimuli produce periodical processes in the plant, which are not imme- 
diately arrested when the stimulus is withdrawn, and only become 
uniform gradually and after the lapse of a considerable time. But I 
certainly claim the right to ask what connexion there is between these 
facts and the transmission of acquired characters. All these peculiar- 
ities produced by external influences remain restricted to the indi- 
vidual in which they arose ; most of them disappear comparatively 
soon, and long before the death of the individual. No example of the 
transmission of such a peculiarity is known. Although successive 
generations of sunflowers have been exposed for thousands of years 
to the daily alternation of light and darkness, the periodicity in the 
flow of sap has not become hereditary, and does not take place at 
all in plants which have always been kept in darkness. Detmer 
specially tells us that we can even reverse the periods of opening 
and closing the leaves in Mimosa jmdica by keeping them in dark- 
ness during the day, but exposed to light at night ; an experiment 
which was performed by Pfeffer. Here again we see the proof 
that influences which have acted upon countless generations have 
left no impression whatever upon the germ-plasm. 

Detmer himself admits this when he says that the after-eflects 
are only witnessed during the life of the individual, but he never- 
theless adds that he has been for many yeai-s convinced that the 
phenomena of heredity and after-efi'ect differ in degree and not in 
kind. He even goes so far as to assert that, in spite of the obvious 
non-transmission of after-effect, the similarity between the natures 
of these two classes of phenomena cannot escape the intelligent 
observer. 

It seems to me that this question does not demand the attention 



TRANSMISSION OF ACQUIRED CHARACTERS. 405 

of the observer (for the observations have already been made) so 
much as that of the thinker. It is not a correct train of reasoning* 
to conckide that after-effect and heredity are identical in nature, 
from the fact that certain periodical influences, acting* upon a single 
individual, set up periodical physiological processes which continue 
for a time after the influences have ceased to act. AVe might 
almost as well argue that the oscillations of a pendulum, which 
continue as after-eflects when the pendulum has been set going, 
are of an identical nature with the process of heredity. All these 
phenomena have indeed this much in common : — a cause which 
acted at some time in the past, but which is no longer visible at 
the time when the phenomenon appears. But the likeness ends 
here, and the supposed identity in nature merely depends upon wild 
speculation. One difference is very obvious, for the phenomena of 
after-effect gradually cease after the withdrawal of the stimulus, 
just like the oscillations of the pendulum, while the pheno- 
mena of heredity continue without any interruption. As far as 
heredity is concerned the physiological processes of after-effect are 
not disting'uishable from any of the other well-known acquired cha- 
racters which are recognizable as morphological changes. After- 
effects are not transmitted, and compared with this fact but little 
importance can be attached to the use of vague analogies by Detmer, 
who would wish to conclude that heredity is only the after-effect 
of processes which had been set going in the parent organism. 

At the end of his paper Detmer applies the ideas which he has 
gained from the consideration of after-effect to certain phenomena 
in the normal life of plants. He suggests that the periodical 
change of leaf in trees and shrubs may have been produced by the 
direct effect of climate. If branches bearing winter buds are cut 
off" in the autumn and are placed in a hot-house, with their cut 
ends in water, the buds do not at once develope, and months may 
often elapse before they begin to break. He argues that this 
experiment proves that the annual periodicity of the plant no 
longer depends directly upon external influences ; these latter pro- 
duced the periodicity at some earlier time, but it has been gradually 
fixed in the organism by after-effect and heredity (!), so that its 
disappearance does not now take place when the stimulus is with- 
drawn, and changes would only happen very gradually under the 
influence of changed climatic conditions. He considers that this is 



406 ON THE SUPPOSED BOTANICAL PROOFS OF THE 

proved from the fact that our cherry has become an evergreen in 
Ceylon. 

Such are Detmer's opinions, and every one will agree with him 
in believing that the periodical change of leaf in temperate climates 
has been produced in relation to the recurring alternation of summer 
and winter. This is certainly the case, and it cannot be doubted 
that the character has become fixed by heredity. Where, however, 
is the proof that this hereditary character has been produced by the 
direct influence of climate ? What right have we to look upon 
the hereditary appearance of the character as an after-effect of the 
direct influence exerted by changes of temperature upon previous 
generations ? Such an opinion derives but little support from the 
previously described experiments upon after-effect, which showed 
that these phenomena were never hereditary. 

It appears to me that there are cei-tain points in this change of 
leaf and its accompanying phenomena, which distinctly indicate 
that natural selection has been at work. Can Detmer imagine that 
the brown scales which form the characteristic protective covering 
of winter buds have been produced by the direct action of the cold? 
If, however, the peculiar structure of these buds is to be referred to 
the specific constitution of the individual rather than to the direct 
effects of climate, would it be so very improbable for their physio- 
logical peciiliarity of lying dormant for several months to have 
been developed simultaneously with the structure, by the operation 
of natural selection? And if this explanation be correct, we can at 
once see why the character has become hereditary, for natural 
selection works upon variations of the germ-plasm, and these are 
transferred from one generation to another with the germ-plasm 
itself. 

But Detmer attempts to establish the converse conclusion, and 
he argues that the hereditary change of leaf has been abandoned 
under the long-continued effect of changed climatic conditions ; 
but this opinion is based upon the single instance of the alteration 
in the habit of the European cherry in Ceylon. If it were proved 
that our cherry, grown from seed in Ceylon and propagated by 
seed for several generations, became evergreen gradually and not 
suddenly in the first generation: if, under such circumstances, it 
came to retain its leaves in the autumn and ceased to produce the 
dormant winter buds; — then indeed the transmission of acquired 



TRANSMISSION OF ACQUIRED CHARACTERS. 407 

characters could hardly be doubted. I am not a botanist, but I 
believe I am right in supposing* that the wild cherry reproduces 
itself by seeds, while the edible domesticated cherry is propagated 
by grafting. Grafts are, however, parts of the soma of a previously 
existent tree, and we are not therefore concerned, in this method 
of propagation, with a succession of generations, but with the suc- 
cessive distribution of one and the same individual over many wild 
stocks. But no one will doubt that one and the same individual 
can be gradually changed dming the course of its life, by the 
direct action of external influences. The really doubtful point is 
whether such changes can be transmitted by means of the germ- 
cells. If, as I presume, the English in Ceylon do not care to eat 
wild cherries but prefer the cultivated kinds, it follows that the 
branches which bear fruit in that island have not been developed 
from germ-cells, at any time since their introduction, and there is 
nothing to prevent them from gradually changing their anatomical 
and physiological characters in consequence of the direct influence 
of climate. 

Hence the instance which Detmer looks upon as plainly con- 
clusive, can hardly be accepted in support of such a far-reaching 
assumption as the transmission of acquired characters. 

It is therefore clear that none of the facts brought forward by 
Detmer really afford the proofs which he believes that they offer. 
But another botanist, Professor Hoffman of Marburg, well known 
for his long-continued experiments on variation, has recently called 
attention to certain other botanical facts in support of the trans- 
mission of acquired characters. These facts are indeed conclusive, 
if we accept the author's use of the term ' acquired,' but it will 
be found that they lead to hardly any modification in the state 
of existing opinion upon the subject. 

In a short note, dated Jan. i, 1888, the author communicated to 
this journal (' Biologisches Centralblatt ') the statement that changes 
in the structure of flowers caused by poor nutrition can be proved 
to be hereditary to a greater or less extent ^. 

A more elaborate account of the experiments will be found in 
several numbers of the 'Botanische Zeitung,' and the author 
expresses his final results in the following words (see Bot. Zeit. 
1887, p. 773) : — 'These experiments prove with certainty (i) that 

' Compare Biol. Centralbl. Bd. VII. No. 21, 



408 ON THE SUPPOSED BOTANICAL PROOFS OF THE 

insufficient nutrition may cause considerable morphological chang-es 
(viz. qualitative variations) which are in the first place acquired 
by thes exual apparatus of the flower, (2) that the "transient" (Weis- 
mann) characters acquired by the individual can be transmitted ^.' 

The data upon which Hoffman bases these opinions are certain 
experiments conducted upon various plants, in order to determine 
the conditions of life under which abnormal flowers or any other 
variations occur most frequently : to decide, in short, how far 
variations are caused by the change of conditions. 

It is obvious that the attention of the author was not at first 
directed to the question of the transmission of acquired characters. 
His experiments are of a much older date than the present con- 
dition and significance of the question before us. Hoffmann has, 
in fact, re-examined his former results from the new point of view, 
and this explains why his proofs are not always sufficiently con- 
vincing when applied to the present issue. But this is of no 
great importance, inasmuch as there is no necessity for me to 
question the correctness of his assumptions. 

The essential details of the experiments to which he directs 
attention are as follows. 

Different plants with normal flowers were subjected to greatly 
changed conditions of life for a series of generations. They were, 
for example, crowded together in small pots. Under these cir- 
cumstances the plants were of course poorly nourished, and in the 
course of generations, several species produced a variable proportion 
of abnormal — viz. double-flowers. This, however, was not always 
the case, for such flowers did not appear in Matthiola annua and 
Helianthertmni j>olifolium. In other species, such as Nigella danias- 
cena, Papaver alpinum and Tagetes patiila, they appeared and often 
increased in numbers in the course of generations, although this 
was not a constant result. For instance, four successive genera- 
tions of Nigella damascena, when closely sown, produced the 
following results : — 

1883. No double flowers. 

1884- » „ . „ 

1885. 23 typical flowers: 6 double flowers. 

1886. 10 ,, „ : I ,, flower. 

• I have used the expression 'transient' ('passant') in the same sense as 
' acquired,' in order to enforce the conclusion that they are merely temporary, and 
disappear with the individual in which they arise. Since the characters of which 



TRANSMISSION OB" ACQUIRED CHARACTERS. 409 

But it was not always the case that the double flowers continued 
to appear after they had been once produced. In Tapaver alpimim, 
which Hoffman has cultivated in successive generations since 
1863, other chang^es in addition to the doubling- of the flowers first 
appeared in 1883, viz. a slight variability in the form of the leaf, 
and a greater variability in the colours of the flowers. The pro- 
duction of double flowers appeared to be favoured by poor nutrition 
caused by crowding the plants. The results as regards the number 
of double flowers produced in this species by close sowing, from 
1883-1886, have been as follows : — 

40 per cent, of double flowers. 



Experiment XI. 


1881. 


40 




1882. 


4 




1883. 


5-3 


Experiment XVII. 


1884. 


13- 




1885. 


00 




i886. 


o-o 



Although in these and some other series of generations the 
double flowers again disappeared in the later generations, yet there 
can be hardly any doubt that their first appearance was due to the 
abnormal conditions of nutrition. This conclusion is also unaffected 
by the fact that double flowers appeared in nearly the same pro- 
portions in consequence of cultivation in ordinary garden soil. The 
plants which were crowded in pots produced 3879 normal flowers, 
and 256 ( = 8-8 per cent.) abnormal and mostly double ones, while 
867 normal and 63 { = 'J-o per cent.) abnormal ones were produced 
on garden beds. Hoffman will not indeed admit that such a 
comparison can be fairly made, for the plants in the garden beds 
were raised from seed which was in part taken from the double 
flowers, and was therefore, he believed, under a strong hereditary 
influence. But this latter assumption is not supported by the 
results of his own experiments. 

Thus experiment XVIII., conducted upon Papaver aljnnum, is 
described in these words, — ' Seeds yielded by double flowers from 
experiment XI. (1883), were sown in pots, and the resulting plants 
produced from 1884-1886, fifty-three single flowers and no double 
ones.' 

HofFmann speaks are hereditary, the term cannot be rightly applied to them, and 
I shall prove later on that they cannot be regarded as acquired characters in the 
sense required by the theory of descent. 



410 ON THE SUPPOSED BOTANICAL PROOFS OF THE 

In the converse experiment XIX. ' The seeds of sing-le flowers 
from different stocks were sown in pots, and the resulting plants 
produced in 1885 and 1886 forty-three flowers, of which all were 
typical except one ; ' while plants produced in the garden by seed 
from the same sources, yielded 166 sing-le and five double flowers. 
Hoffman also describes other experiments in which the seeds from 
double flowers produced plants which also yielded many double 
flowers. Thus, for example, in experiment XXI. seeds yielded by 
the double flowers of Papaver alpinum were sown in the garden 
and produced numerous plants, which in 1885 and 1886 bore 284 
single and twenty-one double flowers, that is 7 per cent, of the 
latter. 

It will therefore be seen that the transmission of the abnormality 
is by no means proved beyond the possibility of doubt, for who can 
decide between the effects due to heredity and changed conditions 
in the last experiment ? I have no doubt however that the results 
are at any rate in part due to the operation of heredity, for I do not 
see how the phenomena can be otherwise understood. Nevertheless 
I cannot admit the transmission of acquired chai-acters on this 
evidence, for the changes which have appeared are not ' acquired ' 
in the sense in which I use the term and in the sense required by 
the general theory of evolution. It is true that they may be 
described by the use of this word : inasmuch as they are characters 
which the plant has come to possess ; we are not however en- 
gaged in a mere dispute about terms, but in the discussion of a 
weighty scientific question. Our object is to decide whether 
changes in the sotna (the body, as opposed to the germ-cells) 
which have been produced by the direct action of external in- 
fluences, including use and disuse, can be transmitted ; whether 
they can influence the germ-cells in such a manner that the latter 
will cause the spontaneous appearance of corresponding changes 
in the next generation. This is the question which demands an 
answer ; and, as has been shown above, such an answer would decide 
whether the Lamarckian principle of transformation must be 
retained or abandoned. 

I have never doubted about the transmission of changes which 
depend upon an alteration in the gerai-plasm of the reproductive 
cells, for I have always asserted that these changes, and these alone, 
must be transmitted. If any one makes the contrary assertion, 



TRANSMISSION OP ACQUIRED CHARACTERS. 411 

he merely proves that he does not understand what I have said 
upon the subject. In what other way could the transformation 
of species be produced, if changes in the g-erm-plasm cannot be 
transmitted ? And how could the g-erm-plasm be changed except 
by the operation of external influences, using the words in their 
widest sense ; unless indeed we assume with Nageli, that changes 
occur from internal causes, and imagine that the phyletic develop- 
ment of the organic world was planned in the molecular structure 
of the first and simplest organism, so that all forms of life were 
compelled to arise from it, in the course of time, and would have 
arisen under any conditions of life. This is the outcome of Nageli's 
view, against which I have contended for years. 

If we now use the term ' acquired characters ' for changes in the 
soma which, like spontaneous abnormalities, depend upon previous 
changes in the germ-plasm — it is of course easy to prove that 
acquired characters are transmitted ; but this is hardly the way to 
advance science, for nothing but confusion would be produced by 
such a use of terms ^. I am not aware that any one has ever 
doubted that spontaneous characters, such as extra fingers or toes, 
patches of grey hair, moles, etc., can be transmitted. It is true 
that such characters are sometimes called ' acquired ' in pathological 
works, but His has rightly insisted that such an obviously inaccurate 
use of the term ought to be avoided, in order to prevent mis- 
understanding. If every new character is said to be ' acquired ' 

^ Compare a paper by J. Orth, 'Ueber die Entstehung und Vererbimg indi- 
vidueller Eigenschaften,' Leipzig, 1887. This author considers my theory of the 
non-transmission of acquired characters to be incorrect, because he will insist upon 
using the term ' acquired ' for those characters which are due to spontaneous changes 
in the germ ; although he considers that they are only indirectly acquired. He also 
reproaches me with not having discriminated with sufficient clearness between the 
two modes in which new characters are acquired by the body, and with having 
altogether failed to take into account the class of characters which are due to 
variations in the germ. On the very same page he quotes the following sentence 
from my writings: — 'Every change of the germ-plasm itself, however it may have 
arisen, must be transmitted to the following generation by the continuity of the 
germ-plasm ; and hence also any changes in the soma w^hich arise from the germ- 
plasm must be transmitted to the following generation.' Not only does the trans- 
mission of Orth's ' indirectly acquired characters ' necessarily follow from this sen- 
tence, but it is even distinctly asserted by it. I cannot understand how any one 
who is aware of what happened at the meeting of the Association of German 
naturalists at Strassburg in 1885, can charge me with the confusion of ideas which 
has prevailed since Virchow took part in the discussion of this question. 



412 ON THE SUPPOSED BOTANICAL PROOFS OP THE 

the term at once loses its scientific value, which lies in the restricted 
use. If generally used, it would mean no more than the word 
*new'; but new characters may arise in various ways, — by artificial 
or natural selection, by the spontaneous variations of the germ, or 
by the direct effect of external influences upon the body, including 
the use and disuse of parts. If we assume that these latter 
characters are transmitted, the further ' assumption of complicated 
relations between the organs and the essential substance of the 
germ becomes necessary ' (His), while the transmission of the 
other kinds of characters do not involve any theoretical difficulties. 
There is therefore obviously a wide difference between these two 
groups of characters as far as heredity is concerned, quite apart 
from the question as to whether acquired characters are really 
transmitted. It is at all events necessary to have distinct terms 
which cannot be misunderstood. His ^ has proposed to call those 
characters which are due to selection 'changes produced by breeding' 
('erziichtete Abiinderungen '), those which appear spontaneously — 
* spontaneous changes ' (' eingesprengte Abiinderungen '), and these 
two groups of characters would then be opposed to those which he 
calls 'acquired changes' ('erworbene Abanderungen '), of course 
using the term in the restricted sense. Science has always claimed 
the right of taking certain expressions and applying them in a 
special sense, and I see no reason why it should not exercise this 
right in the case of the term ' acquired.' It appears moreover that 
this word has not always been used in this vague sense by patho- 
logical anatomists, such as Virchow and Orth ; for Weigert and 
Ernst Ziegler have employed it in precisely the same sense as that 
in which it has been used by Darwin, du Bois-Reymond, Pfliiger, 
His and many others, including myself. 

It is certainly necessary to have two terms which distinguish 
sharply between the two chief groups of characters — the primary 
characters which first appear in the body itself, and the secondary 
ones which owe their appearance to variations in the germ, how- 
ever such variations may have arisen. We have hitherto been 
accustomed to call the former ' acquired characters,' but we might 
also call them * somatogenic^ because they follow from the reaction 
of the soma under external influences ; while all other characters 
might be contrasted as ' hlasiogemcl because they include all those 
* His, ' Unsere Korperform,' Leipzig, 1874, p. 58. 



TRANSMISSION OF ACQUIEED CHARACTERS. 413 

characters in the body which have arisen from chang-es in the 
germ. In this way we mig-ht perhaps prevent the possibility of 
misunderstanding'. We maintain that the ' somatogenic ' characters 
cannot be transmitted, or rather, that those who assert that they 
can be transmitted, must furnish the requisite proofs. The somato- 
genic characters not only include the effects of mutilation, but 
the changes which follow from increased or diminished performance 
of function, and those which are directly due to nutrition and any 
of the other external influences which act upon the body. Among 
the blastogenic characters, we include not only all the changes 
produced by natural selection operating upon variations in the 
germ, but all other characters which result from this latter 
cause. 

If we now wish to place Hoffmann's results in their right 
position, we must regard all of them as ' blastogenic ' characters, 
for no one of them can be considered as belonging to the group 
which has been hitherto spoken of as ' acquired,' in the literature 
of evolution : they are not due to somatogenic but to blastogenic 
changes. The body of the plant — the soma — has not been directly 
affected by external influences, in Hoffman's experiments, but 
changes have been wrought in the germ-plasm of the germ-cells 
and, only after this, in the soma of succeeding generations. 

There is no difficulty in finding facts in support of this state- 
ment, among Hoffmann's experiments. The proof chiefly lies in 
the fact that in no one of his numerous experiments did any 
change appear in the first generation. The seeds of different 
species of wild plants, with normal flowers, were cultivated in 
the garden and in pots (thickly sown in the latter case), but no 
one of the plants produced by these wild seeds possessed a single 
double flower. It was only after a greater or less number of 
generations had elapsed that a variable proportion of double 
flowers appeared, sometimes accompanied by changes in the leaves 
and in the colours of the flowers. This fact admits of only 
one interpretation; — the changed conditions at first produced 
slight and ineffectual changes in the idioplasm of the individual, 
which was transmitted to the following generation : in this again 
the same causes operated and increased the changes in the idioplasm 
which was again handed down. Thus the idioplasm was changed 
more and more, in the course of generations, until at last the 



414 ON THE SUPPOSED BOTANICAL PROOFS OF THE 

change became great enough to produce a visible character in the 
soma developed from it, such as, for example, the appearance of 
a double flower. Now the idioplasm of the first ontogenetic stage 
(viz. germ-plasm) alone passes from one generation to another, 
and hence it is clear that the germ-plasm itself must have been 
gradually changed by the conditions of life until the alteration 
became sufficient to produce changes in the soma^ which appeared 
as visible characters in either the flower or leaf ^. 

In addition to the above-mentioned eases Hoffmann also quotes 
some facts of a somewhat different kind. He succeeded in in- 
ducing considerable changes in the structure of the root of the 
wild carrot [Daucus carota) by means of the changes in nutrition 
implied by garden cultivation. These changes also proved to be 
hereditary. 

Unfortunately, I have not the literature of the subject at hand, 
and hence I am unable to read the accounts of these older experi- 
ments in extenso ; but it is suflSciently obvious that in this case 
we are also concerned with a change which did not become visible 
until after some generations had elapsed, and which was therefore 
a change in the germ-plasm. 

Many instances of a precisely similar kind have been long known, 
and one of them is to be found in the history of the garden pansy, 
which Hoffmann has succeeded in producing from the wild form, 
Viola tricolor, in the course of eighteen years. Darwin some time 
ago pointed out in his work upon ' The Variation of Animals and 
Plants under Domestication,' that, in the case of the pansy and 
all other ' improved ' garden flowers, the wild form remained un- 
changed for many generations after its transference to the garden, 
apparently uninfluenced by the new conditions of life. At length 
single varieties began to appear, and these were further developed 
by artificial selection and appropriate crossing, into well-marked 
races distinguished by peculiar colours, forms, etc. 

In these cases also, changes in the germ-plasm are the first 

^ Compare on this point Nageli in his ' Theorie der Abstainmungslelire.' This 
writer also concludes from similar facts that external influences have wrought in 
the idioplasm, changes which were at first ineffectual, and which only increased 
during the couree of generations up to a point at which they could produce visible 
changes in the plant. He does not, however, draw the further conclusion that these 
changes only influence the germ-plasm, for he was not aware of the distinction between 
germ-plasm and somatoplasin. 



TRANSMISSION OF ACQUIEED CHARACTEKS. 415 

results of the new conditions, and there is no evidence for the 
occurrence of acquired characters, using the term in its restricted 
sense. 

I now come to the last botanical fact brought forward by 
Hoffmann in support of the transmission of acquired characters. 
He states that specimens of Solidago virgaurea broug-ht from the 
Alps of the Valais, commenced flowering- in the botanical g-arden 
at Giessen, at a time which differed by several weeks from that 
at which specimens from the surrounding* country, planted beside 
them, began to flower. In other words, the time of flowering 
must have been fixed by heredity in the alpine Solidago, for the 
external conditions would have favoured a time which was simul- 
taneous with that of the Giessen plants. 

What conclusions can be drawn from these facts ? Hoffmann of 
course sees in them the proof of the transmission of acquired 
characters, but this presupposes that the time of flowering was 
originally an acquired character. Hoffmann indeed appears to 
entertain this opinion when he somewhat vaguely states that the 
time at which flowering begins has been acquired by accommodation 
— that is by the influence of climate — during a long series of 
generations, and has become hereditary. But what does Hoffmann 
mean by ' accommodation ' ? He presumably means that which, 
since the appearance of Darwin's writings, has been generally 
called adaptation : — that is a purposeful arrangement, suited to 
certain conditions. The majority of biologists have foUowed 
Darwin in believing that such adaptations have been produced by 
processes of natural selection. Hoffmann seems to imagine that 
they have arisen in some other way : perhaps he believes, with 
Nageli, that they have been directly produced by external in- 
fluences. 

The fixation of the time at which flowering begins, is an 
adaptation which formerly could have been very well explained 
as the direct result of external conditions. The question we have 
to decide is whether such an explanation is the true one. We 
might imagine that the plant would be forced into quicker develop- 
ment by an eai'lier appearance of the warm season. Hence when 
transferred into a warmer climate the plant would at first flower 
rather earlier, the habit would then be transmitted, and would 
increase in successive generations from the continued influence 



416 ON THE SUPPOSED BOTANICAL PROOFS OF THE 

of climate, until it advanced as far as the organization of the 
plant permitted. But in this explanation, as in so many others 
of the same kind, it has unfortunately been forgotten that the 
transmission of acquired characters which is presupposed in the 
explanation is a totally unproved hypothesis. It is sufficiently 
obvious that by interpreting a phenomenon in a manner which 
presupposes the transmission of acquired characters, we cannot 
furnish a proof of the existence of such transmission. 

It always seemed to me that the fixation of the commencement 
of flowering, together with similar physiological phenomena in the 
animal kingdom (for example, the hatching of insects from winter 
eggs), could be explained very satisfactorily by the operation of 
natural selection : and even now this explanation appears to me 
to be the simplest and most natural. In Freibnrg, where the 
vine is largely grown, the harvest is often injured by frosts in 
spring, which kill the young shoots, buds and flowers. Accord- 
ingly, different kinds of vine, which do not push their buds so 
early, have now been planted. Any one, who has seen all the 
shoots of the former destroyed by the frosts at the end of April, 
while the latter, not having opened their buds, were spared, 
would not doubt that the former must have been long ago 
exterminated, if they had been compelled to struggle for existence 
with the others, under natural conditions. Now the time of 
flowering fluctuates slightly in the individuals of every species of 
plant, and can therefore be modified by natural selection. It is 
therefore difficult to see why the time at which each plant flowers 
should not have been flxed in the most favourable manner for each 
habitat, by natural selection alone. 

Hofimann is obviously unaware of the fundamental distinction 
between the characters primarily acquired by the soma, and the 
secondary characters which follow from changes in the germ- 
plasm. 

If the author had appreciated this distinction he would not 
have attempted to strengthen his opinions by following up the 
botanical facts which exclusively belong to the second class of 
characters, with the enumeration of certain instances selected from 
the animal kingdom (viz., the supposed transmission of mutila- 
tions), all of which belong to the first class. I v^ll not discuss 
these latter instances, for most of them are old friends, and they 



TRANSMISSION OF ACQUIRED CHARACTERS. 417 

are all far too uncertain and inaccurate to have any claim on 
scientific consideration. 

I believe that I have shown that no botanical facts have been 
hitherto brought forward which prove the transmission of acquired 
characters (in the restricted sense), and that there are not even 
any facts which render such transmission probable. 



A. W. 



Naples, Zoological Station, 
Jan. II, 1888. 



V. e 



VIII. 

THE SUPPOSED TRANSMISSION OF 
MUTILATIONS. 

1888. 

A lectufe delivered at the Meeting of the Association of German Naturalists 
at Cologne, September 1888. 



E e 2 



VIII. 



THE SUPPOSED TRANSMISSION OF 
MUTILATIONS. 

We know well tlie manner in which Lamarck imagined that the 
gradual transformation of species occurred, when he first made 
the attempt to penetrate into the mechanism of the process of 
evolution, and to ascertain the causes by which it is produced. In 
his opinion, a change in the structure of any part of an organism 
was chiefly brought about when the species in question met with 
new conditions of life and was thus forced to assume new habits. 
Such habits caused an increased or diminished activity, and there- 
fore a stronger or weaker development, of certain parts, and the 
modified parts were then transmitted to the offspring. Inasmuch 
as the off^spring continued to live under the same changed conditions, 
and kept up the altered manner of using the pai*t in question, the 
inherited changes would be increased in the same direction during 
the course of their life, and would be further increased in each 
successive generation, until the greatest possible change had been 
effected. 

In this way Lamarck was able to give an apparently satisfactory 
explanation of at any i-ate those changes which consist in the mere 
enlargement or diminution of a part ; such, for instance, as the 
great length of neck in the swan and other sw